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Trans. xcvii. 249. Known quantities of the air to be tried, and of nitrous gas being mixed, were admitted......into a graduated tube, which he [Priestley] denominated a eudiometer." This seems to point directly to Priestley as the author of the name as he certainly was the author of the process. (It may be mentioned in passing that, in this paper, Pepys describes the method of calibrating eudiometers, by pouring in equal quantities of mercury from a tube closed at one end and with the mouth ground flat, against which a piece of plate glass is pressed in order to obtain an exact measure of the mercury.)

With these directions I searched in the library of the Royal Society and found Magellan's book; but he uses the name eudiometer as if it were well known. Mr. White, the librarian, very kindly interested himself in the matter and found in Priestley's book, "Observations on different kinds of Air," a statement that he had received from Landriani one of his eudiometers together with a description that he asks Priestley to print, but the latter excuses himself on the ground that it would not be convenient for him to publish the letter at that time. Mr. White found the title of a book by Marsilio Landriani, "Ricerche fisiche intorno alla salubrità dell' aria" (Milano, 1775, 8°). It is not in the libraries of the Royal or of the Chemical Society, and the title does not appear in the catalogue of the library of the Royal Institution, but last week I found the book at the British Museum. On page viii. of the Intro duction there is a paragraph of which the following is a translation: "The account of the discovery of nitrous air and of some of its principal properties is briefly set forth, certain defects of Priestley's apparatus are removed, and there is added a detailed description of the Eudiometer, for that is the name which I give to my little instrument, from Eudios, a Greek word signifying goodness of the air (bontà dell' aria) accompanied by the more useful precautions for its construction.' There are some plates at the end of the book containing drawings of the apparatus, and one of them is marked "Eudiometro 1775." This seems to leave it without a doubt that it is to Landriani that we owe the word.

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Next as to its exact meaning: by tradition we have been taught that the eudiometer is an apparatus for measuring the goodness of air, and this is obviously what was intended by Landriani. The New English Dictionary derives it from edios clear (weather) and μérpov; Roscoe and Schorlemmer derive it from evdla, fine weather, and μérpov; these meanings of the Greek words are no doubt correct, and the name would seem to be more applicable to some kind of weather glass, a signification which the above quotation shows could hardly have been in Landriani's mind. HERBERT MCLEOD.

Cooper's Hill, March 21.

Blind Animals in Caves.

ALTHOUGH in my previous letter I did not give evidence directly supporting the proposition that blind cave-animals are born or hatched with relatively well-developed eyes, that thesis is a good deal more than a mere supposition, as Prof. Lankester calls it. Nor did I, as Prof. Lankester asserts, proceed to state that no such fact is known or recorded. The condition of the eyes in the newly-born young of the viviparous Amblyopsis, or other cave-fishes, does not appear to have been investigated, although living young were born under observation as long ago as 1844, and exhibited as spirit specimens to the Belfast Society of Natural History. Nor have the early stages of the European Proteus been obtained. But, on the other hand, with respect to cave crustacea, Tellkampf, the original describer of the blind Cambarus pellucidus of the mammoth cave, stated that the eyes were larger in the young than in the adult (A. S. Packard, Amer. Nat. 1871), and Garman (Bull. Mus. Comp. Zool xvii. 1888-89) states that in very young specimens of C. setosus, the blind crayfish of the Missouri caves, "the eyes are more prominent, and appear to lack but the pigment." In another blind subterranean species, Troglocaris Schmidtii, occurring in Central Europe, Dr. Gustav Joseph found and demonstrated that the embryo in the egg was provided with eyes. (See Packard, "Cave Fauna of N. America," Nat. Acad. Sci. vol. iv. Mem. 1.)

Thus, although it is obvious enough that further investigation of the development of cave-animals is required, it cannot be said that it is altogether a "hitherto unattempted embryological research." A discussion of this kind ought not, however, to be

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The Value of the Mechanical Equivalent of Heat. IN NATURE for March 16 you published a summary of a communication which I had the honour to make to the

Royal Society. My conclusion as to the value of the C.G.S. unit of heat was 4'1940 × 107 ergs (see NATURE, p. 478), and I added the following comment : 'If we express Rowland's result in terms of our thermal unit we exceed his value by I part in 930, and we exceed the mean value of Joule's (selected) determinations by one part in 350, .. if we attach equal value to all the results published by Joule his value exceeds ours by I part in 4280."

I have received so many communications with regard to this last statement, that you will perhaps permit me to answer my correspondents through your columns.

I thought it unnecessary in a short summary to point out that the value (in gravitation and Fahrenheit units) resulting from Joule's own experiments is not the usually accepted 772 55. To me it appears an extraordinary thing that 772 is to this day given in the text-books when, so far back as 1880, Rowland conclusively proved that the results obtained from Joule's experiments give a higher value (see Proceedings, American Academy, March 1880).

In 1879 Rowland forwarded to Joule a thermometer by Baudin, which had been directly compared with Rowland's air thermometer. Joule himself then made a careful comparison of his thermometer with the Baudin one, and communicated the results to Rowland. The complete table is given on p. 39. of the paper already referred to. In addition to the correction thus shown to be necessary, further corrections for the capacity for heat of the calorimeter and contents were included, and as the results were published in Joule's lifetime, there can be little doubt but that these corrections received his approval.

I give an example (from p. 44) of Rowland's corrections :

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It is evident that Rowland did not claim for his air thermometry an order of accuracy greater than or°. In the appendix to his paper (p. 197) he remarks that if a certain improvement (not then adopted by him) was made, "it is probable that an accuracy of or C. could be obtained from the mean of two or three observations. I believe that my own thermometers scarcely differ much more than that from the absolute scale at any point up to 40° C."

A study of Rowland's methods, and of the tables given in his admirable paper, leads to the conclusion that it is possible that his thermometry is in error by I in 1000 over the range 15° to 25°, and such an error would suffice to bring together the results (both in the value of J and in the temperature coefficient of the specific heat of water) obtained by Rowland and myself. The error would, however, but slightly affect the correction of Joule's results.

If we attach arbitrary values to Joule's later experiments, the mean of the corrected values (by Rowland's thermometer) is 776 75 (32195); and the mean of all his determinations by various methods is 779'17,1 and we may regard the above as within 1 in 1000 of the value resulting from Joule's own work on this subject.

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I trust that in future our engineers and text-book writers will (even if they ignore the work of later observers) do Joule the justice of discarding the traditional 772, and adopt a value more in harmony with the investigations of that great experimentalist. E. H. GRIFFITHS.

12, Parkside, Cambridge.

1 In terms of a thermal unit at 15° C.

THE SENSITIVENESS OF THE EYE TO LIGHT AND COLOUR.1

THERE may be some here who have had the pleasure

-or the pain-of rising very much betimes in a Swiss centre of mountaineering in order to gain some mountain peak before the sun has had power enough to render the intervening snow-fields soft, or perhaps dangerous. Those who have will recollect what were the sensations they experienced as they sallied out of the comfortable hotel, after endeavouring to swallow down breakfast at 2 a.m., into the darkness outside. Perhaps the night may have been moonless, or the sky slightly overcast, and the sole light which greeted them have been the nervous glimmer of the guides' lanterns. By this feeble light they may have picked their way over the stony path, and between the frequent stumbles over some half hidden piece of rock lying in the short grass they may have had time to look around and above them, and notice that the darkness of the night was alone broken by stars which gave a twinkle through a gap in the clouds, or if the sky were cloudless, every star would be seen to lie on a very slightly illuminated sky of transparent blackness. Although giant mountains may have been immediately in front of them, their outlines would be almost if not quite invisible. As time went on the sky would become a little brighter, and what is termed the petit Jour would be known to be approaching. The outlines of the mountains beyond would become fairly visible, the tufts of grass and the flowers along the path would still be indistinguishable, and most things would be of a cold grey, absolutely without colour. The guide's red woollen scarf which he bound round his neck and mouth would be black as coal. But a little more light, and then some flowers amongst the grass would appear as a brighter grey, though the grass itself would still appear dark; but that red scarf would still be as black as a funeral garment. The mountains would have no colour. The sky would look leaden, and were it not for the stars above it might be a matter of guesswork whether it were not -covered over with cloud.

More light still, and the sky would begin to blush in the part where the sun was going to rise, and the rest would appear as a blue-grey; the blue flowers will now be blue, and the white ones white; the violet or lavender coloured ones will still appear of no particular colour, and the grass will look a green grey, whilst the guide's neckgear will appear a dull brown.

The sun will be near rising, the white peaks beyond will appear tipped with rose; every colour will now be distinguished, though they would still be dull; and, finally, the daylight will come of its usual character, and the cold grey will give place to warmth of hue.

But there may be others who have never experienced this early rising, and prefer the comfort of an ordinary English tramp to that just described; but even then they may have felt something of the kind. In the soft autumn evening, when the sun has set, they may have wandered into the garden and noticed that flowers which in the daytime appear of gorgeous colourings-perhaps a mixture of red and blue-in the gloaming will be very different in aspect. The red flowers will appear dull and black; a red geranium, for instance, in very dull light, being a sable black, whilst the blue flowers will appear whitishgrey, and the brightest pale yellow flowers of the same tint; the grass will be grey, and the green of the trees the same nondescript colour. A similar kind of colouring will also be visible in moonlight when daylight has entirely disappeared, though the sky will have a transparent dark blue look about it, approaching to green. These sensations, or rather lack of sensations of light and

1 A Lecture delivered a the Roval Institution of Great Britain by Cap ain W de W. Abney, C. B., R. E., DC.L, F.RS.

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colour, which as a rule attract very little attention, as they are common ones, are the subjects of my discourse to-night.

Experiments which can be shown to a large audience on this subject are naturally rather few in number, but I will try and show you one or two.

We are often told that the different stages of heat to which a body can be raised are black, red, yellow, and white heat, but I wish to show you that there is an intermediate stage between black and red heat, viz. a grey heat. An incandescent lamp surrounded by a tissue paper shade, has a current flowing through it, and in this absolutely dark room nothing is seen, for it is black hot An increase of the current, however, shows the shade of a dim grey, whilst a further increase shows it as illuminated by a red, and then a yellow light. A bunch of flowers placed in the beam of the electric light shows every colour in perfection; the light is gradually dimmed down, and the reds disappear, whilst the blue colours remain and the green leaves become dark. These two experiments show that there is a colour, if grey may be called a colour. with which we have to reckon.

Now the question arises whether we can by any means ascertain at what stage a colour becomes of this grey hue, and at what stage of illumination the impression of mere light also disappears, and whether in any case the two disappear simultaneously.

As all colours in nature are mixed colours, it is at the outset useless to experiment with them in order to arrive at any definite conclusion, hence we are forced-and the forcing in this direction to the experimentalist is a very agreeable process-we are forced to come to the spectrum

for information.

The apparatus on this table is one which I have before described in this theatre, and it is needless for me to describe it again. I can only say that it has in all colour investigations been of such service that any attempt on my part to do without it would have been most disadvantageous. The apparatus enables a patch of what is practically pure monochromatic light of any spectrum colour to be placed upon the screen at once, and an equally large patch of white light alongside it, by means of the beam reflected from the first surface of the first prism.

It should be pointed out that this beam of white light reflected from the first prism of the apparatus, having first passed through the collimator, must of necessity diminish with the intensity of the spectrum, when the collimator slit is closed.

Having got these patches, the next step is to so enfeeble the light that their colour and then their visible illumination disappear.

An experiment which well demonstrates loss of colour is made by throwing a feeble white light on one part of the screen, and then in succession patches of red, green. and violet alongside it. The luminosity of the coloured light gradually diminishes till all the colour disappears, the white patch being a comparison for the loss of colour.

If red, green, and violet patches be placed alongside each other, and they are bedimmed in brightness together, it will be noticed that the red disappears first, then the green, and then the violet; or I may take a red and green patch overlapping, which when mixed form orange, and extinguish the colour: the slit allowing red light to fall on the screen may be absolutely closed, and no alteration in the appearance of the patch is found to occur. This shows, I think, that when all colour is gone from a once brilliant colour, a sort of steel-grey remains behind, and that red fails to show any luminosity when the green still retains its colour.

The measurement of the extinction of colour from the different parts of the spectrum was made on these prin

iples. A box, similar to Fig. 2, was prepared, but having wo apertures, one at each side. Through one the oloured ray was reflected, and through the other a white

out for the ordinates); each curve is therefore madeon a scale ten times that of its neighbour, counting from the centre.

In the diagram the sodium light of the spectrum before extinction was made of the luminosity of the amylacetate lamp (hereafter called AL), which is about 8 of a standard candle, at I foot distance from the source. Before it ceased to cause an impression on the eye, the illumination had to be reduced 350 10,000,000

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A L.

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FIG. 1.-Extinction of Spectrum Colours.

beam of light to a white screen. Both beams were diminished, and when the white and coloured patches appeared the same hue, the amount of illumination was calculated. Fig. 1 shows graphically the reduction of illumination, when the D light of the spectrum is the same intensity as one amyl-acetate lamp at one foot from the screen. To measure the extinction of light, a box was made as in the diagram, closed at each end, but having two apertures as shown, Fig. 2: -E is a tube through which the eye looks at S, which is a black screen with a white spot upon it, and which can be illuminated by light coming through the diaphragm D first falling on a ground glass which closes the aperture, and reflected on to it by M a mirror.

The patch of light of any colour being thrown on D,rotating sectors, the apertures of which could be opened and closed at pleasure, were placed in the path of the beam, thus enabling the intensity of the patch to be diminished. D could be made of any desired aperture, and thus the illumination of the ground glass would be diminished at pleasure. After keeping the eye in darkness for some time, the eye was placed at E, when the white spot illumi nated by the colour thrown on D was visible, and the sectors closed till the last scintilla of light was extinguished. This was repeated for rays at different parts of the

spectrum, and the results are shown in Fig. 3 by the continuous curved lines. The diagram would have been too large had the same scale been adopted through

There was one objection which might have been offered to this method, and that was to the use of the rotating sectors, and perhaps to the ground glass. This objection was met by first of all reducing the light by means of a double reflection of the beam forming the patch from one or two plain glass mirrors, and also by using a plain glass mirror in the box instead of a silvered glass. By this plan the light falling on the first plain glass mirror was reduced, before it reached the end of the box, 1000 times; and again, by narrowing the slit of the collimator, and also the slit placed in the spectrum,

FIG. 2.-Extinction Box.

another similar reduction would be effected. All rays thus enfeebled were within the range of extinction. It was found that neither ground glass nor rotating sectors

Of its spectrum luminosity.

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In the curves there are two branches at the violet side, and this requires explanation. One shows the extinction when viewed by the most sensitive part of the eye, wherever that may be, and the other when the central portion of the eye was employed. The explanation of this difference in perception is chiefly as follows:

In the eye we have a defect-at least we are apt to call it a defect, though no doubt Providence has made it for a purpose-in that there is a yellow spot which occupies some 6° to 8° of the very centre of the retina, and as it is on this central part that we receive any small image, it has a very important bearing on all colour experiments. The yellow spot absorbs the blue-green, blue, and violet rays, and exercises its strongest absorption towards the centre, though probably absent in the very centre, that is, in the "fovea centralis," and is less at the outer edges. That absorption of colour by the yellow spot takes place can be shown you in this way. Any colour in nature can be imitated by mixing a red, a green, and violet together, and with these I will make a match with white and then with brown, two very representative colours, if we may call them colours. Now if

I, standing at this lecture table, match a white by mixing these three colours together, using

a large patch, the image will fall on a part of the retina of considerably larger area than the yellow spot, and it will appear too green for those at a distance; but it is correct for myself. If I place a mirror at a distance, and make a match again by the reflected image, the match is complete for us all, as we all see it through the yellow absorbing medium. If I look at it direct from where I stand the match is much too pink. It may be asked why the comparison patches and the mixed colours do not always match since both images are received on the same part of the retina. The reason is that the green I have selected for mixture is in the part of the spectrum where great absorption takes place, whilst the comparison white contains the green of the whole spectrum, some parts of which are much less absorbed than others. I may remark that just outside the yellow spot the eye is less sensitive to the red than is the centre, and this is one additional cause of the difference. See Fig. 5.

More on this subject I have not time to say on this occasion, but it will be seen that the extinction of light for the centre and the outside of the eye differs on account of this.

I must take you to a theory of colour vision which, though it may not be explanatory of everything, at all events explains most phenomena-that is, the YoungHelmholtz theory. The idea embodied in it is that we have three sensations stimulated in the eye, and that these three sensations give an impression of a red, a green, and a violet. These three colours I have said can be mixed to match any other colour, or, in other words, the three sensations are excited in different degrees, in order to produce the sensation of the intermediate spectrum colours, and those of nature as well.

The diagram Fig. 4 shows the three sensations as derived from colour equations made by Koenig. It will be seen that there are three complete colour sensations, all of which are present in the normal eye. I would ask you to note that at each end of the spectrum only one sensation is present, viz. at the red end of the spectrum, the red sensation, and at the violet end the violet.

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FIG. 4.-Colour Sensations.

This is a matter of some importance, as we shall now

see.

It will be recollected that in making the extinctions, the D'light of the spectrum was made equal to one amyl-acetate lamp, and the other rays had the relative luminosity to it, which they had in the spectrum before they were extinguished. The luminosity curve of the spectrum is shown in Fig. 5.

Suppose we make all the luminosities of the different rays equal to one A L., we should not get the same extinction value, as shown in the continuous lines in Fig. 3. The violet would have to be much more reduced, but by multiplying the extinction by the luminosity we should get the curve of reduction for equal luminosities, and we get the dotted curves in Fig. 3.

It will be seen that it is the violet under such circumstances that would be the last to be extinguished, and that all the rays at the violet end of the spectrum would be extinguished simultaneously, as would also those at the extreme red. This looks like a confirmation of the Young-Helm

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This being so, I think it will be pretty apparent that, at all events from the extreme violet to the Fraunhofer line D of the spectrum, the extinction is really the extinction of the violet sensation, a varying amount of which is excited by the different colours. If then we take the reciprocals of the numbers which give extinction of the spectrum, we ought to get the curve of the violet sensation on the Young-Helmholtz theory. For if one violet sensation has to be reduced to a certain degree before it is unperceived, and another has to be reduced to half that amount, it is evident that the violet sensation must be double in one case to what it is in the other; that is, the degrees of stimulation are expressed by the reciprocal of the reduction.

Such a curve is shown in Fig. 5 (in which also are drawn the curves of luminosity of the spectrum when viewed with the centre of the retina and outside the yellow spot). And it will be noticed that it is a mountain which

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would have to be placed only 67 feet away, whilst the radiation for an AL of the colour of the B light of the 2600 spectrum would have to be diminished to but

10 millionths or the screen would have to be placed 60 feet away. It is therefore apparent that with equal luminosities the violet requires about 175 times more reduction to extinguish it than does the red, and probably about 25 times more than the green.

reaches its maximum about E. Remember that the height of the curve signifies the amount of stimulation given to the violet sensatory apparatus by the particular ray indicated in the scale beneath.

Turning once more to Fig. 3, it will be noticed that if any one or two of the three sensations are absent, the persons so affected are, what is called, colour-blind. Thus if the red sensation is absent, they are red-blind; if the green, then green-blind; if the violet, then violet-blind; if both red and green sensations are absent, then the person would see every colour, including white, as violet. The results of the measurement of the luminosity of the spectrum by persons who have this last kind of monochromatic vision should be that they give a curve exactly or at all events very approximately, of the same form as the curve given by the reciprocals of the extinction curve obtained by the normal eye, as the violet sensation is that which is last stimulated.

It has been my good fortune to examine two such persons, and I find that this reasoning is correct, the two coinciding when the curves for the centre of the retina are employed.

Further, I examined a case of violet blindness, and measured the luminosity of the spectrum as apparent to him. Now if the Young-Helmholtz theory be correct, then in his case the violet sensation ought to be absent, and the difference between his luminosity and that of the normal eye ought to give the same curve as that of the violet sensation. This was found to be the case.

Again, the reciprocal of the extinction curves of the red-blind and green-blind ought to be the same as those of the normal eye, for the violet sensation must be present with them also. This was found to be so. We have still one more proof that the last sensation to disappear is the violet.

If we reduce the intensity of the spectrum till the green and red disappear to a normal eye, and measure the luminosity of the spectrum in this condition, we shall find that it also coincides with the persistency curve. On the screen we have a brilliant spectrum, but by closing the slit admitting the light and placing the rotating sectors in the spectrum and nearly closing the apertures, we can reduce it in intensity to any degree we like. The whole spectrum is now of one colour and indistinguishable in hue from a faint white patch thrown above it. If the luminosity of this colourless spectrum be measured we

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