218

PERCEPTION OF ULTRA-VIOLET RAYS.

To our eyes both, as I say, are equally dark, and so they would be to the Daphnias if their limits of vision were the same as ours. As a matter of fact, however, the Daphnias all collected in the part of the trough under the water, and avoided that under the bisulphide of carbon, showing that this, therefore, was to them darker than the other. I varied the experiments in several ways, but always with similar results. Bichromate of potash is also impervious to the ultra-violet rays, and had the same effect.

Not satisfied with this, I tried to test it in another way.

I took a cell, in which I placed a layer of five-percent. solution of chromate of potash less than an eighth of an inch in depth, and which, though almost colourless to our eyes, completely cut off the ultra-violet rays. I then turned my trough at right angles, so that I could cover one side of the ultra-violet portion of the spectrum with the chromate and leave the other exposed. The numbers were as follows:

:

May 19.-I again tried the same arrangement, reducing the chromate of potash to a mere film, which,

OBJECTIONS OF M. MEREJKOWSKY.

219

however, still cut off the ultra-violet rays. I then placed it, as before, over one half of the ultra-violet portion of the spectrum; and over the other half I placed a similar cell containing water. Between each experiment I reversed the position of the two cells. The numbers

Evidently, then, even a film of chromate of potash exercises a very considerable influence; and, indeed, I doubt not that, if a longer time had been allowed, the difference would have been even greater.

It seems clear, therefore, that a five-per cent. solution of chromate of potash only one-eighth of an inch in thickness, which cuts off the ultra-violet rays, though absolutely transparent to our eyes, is by no means so to the Daphnias.

These observations seem to prove, though I differ with great reluctance from so eminent an authority as M. Paul Bert, that the limits of vision of Daphnias do not, at the violet end of the spectrum, coincide with ours, but that the Daphnia, like the ant, is affected by the ultra-violet rays.

Since these observations were published, M. Merejkowsky has experimented on the subject, and come to the conclusion that the Daphnias are attracted wherever there is most light, that they are conscious only of the intensity of the light, and that they have no power of distinguishing colors. It is no doubt true that in ordinary diffused daylight the Daphnias generally

220

DAPHNIAS SUPPOSED TO PERCEIVE

congregate wherever the light is strongest. Their eyes are, however, so delicate that one would naturally expect, à priori, that there would be a limit to this; and, in fact, direct sunshine is somewhat too strong for their comfort.

For instance, I took a porcelain trough, seven and a half inches long, two and a half broad, and one deep, and put in it some water containing fifty Daphnias. One half I exposed to direct sunlight, and the other I shaded, counting the Daphnias from time to time, and transposing the exposed and shaded halves. The numbers were as follows:

This seems clearly to show that they avoid the full sunlight.

I believe, then, that in some of my previous experiments the yellow light was too brilliant for them; and the following experiments seem to show that, when sufficiently diffused, they prefer yellow to white light.

M. Merejkowsky, however, denies to the crustacea any sense of color whatever. His experiments were made with larvæ of Balanus and with a marine copepod, Dias longiremis. These, if I understand him correctly, have given identical results. He considers

BRIGHTNESS, BUT NOT COLOR.

221

that they perceive all the luminous rays, and can distinguish very slight differences of intensity; but that they do not distinguish between different colors. He sums up his observations as follows:

"Il résulte de ces expériences que ce qui agit sur les Crustacés, ce n'est point la qualité de la lumière, c'est exclusivement sa quantité. Autrement dit, les Crustacés inférieurs ont la perception de toute onde lumineuse et de toutes les différences, même très légères, dans son intensité; mais ils ne sont point capables de distinguer la nature des ondes, de différentes couleurs. Ils distinguent très bien l'intensité des vibrations éthérées, leur amplitude, mais point leur nombre. Il y a donc, dans le mode de perception de la lumière, une grande différence entre les Crustacés inférieurs et l'Homme, et même entre eux et les Fourmis; tandis que nous voyons les différentes couleurs et leurs différentes intensités, les Crustacés inférieurs ne voient qu'une seule couleur dans ses différentes variations d'intensité. Nous percevons des couleurs comme couleurs; ils ne les perçoient que comme lumière.” *

It is by no means easy to decide such a question absolutely; but the subject is of much interest, and accordingly I made some further experiments, as it did not seem to me that those of M. Merejkowsky bore out the conclusion he has deduced from them.

Professor Dewar most kindly arranged the apparatus for me again. He prepared a normal diffraction-spectrum, produced by a Rutherfurd grating with 17,000 lines to the inch; the spectrum of the first order was thrown on the trough. In this case the distribution of

* M. C. Merejkowsky, "Les Crustacés inférieurs distinguent-ils les couleurs ?

222

FURTHER EXPERIMENTS.

luminous intensity has been shown to be uniform on each side of the line having the mean wave-length, i.e. a little above the line D in the yellowish green of the spectrum.

I then took a long shallow trough in which were a number of Daphnias, and placed it so that the centre of the trough was at the brightest part of the spectrum, a little, however, if anything, towards the green end. After scattering the Daphnias equably I left them for five minutes, and then put a piece of blackened cardboard over the brightest part. After five minutes more, there were at the green end, 410; in the dark, 14; at the red end, 76. Here the two ends of the trough were equally illuminated; but the preference for the green over the red side was very marked.

I then took five porcelain vessels, seven and a half inches long, two and a half broad, and one deep, and in each I put water containing fifty Daphnias. One half of the water I left uncovered; the other half I covered respectively with an opaque porcelain plate, a solution of aurine (bright yellow), of chlorate of copper (bright green), a piece of red glass, and a piece of blue glass. Every half-hour I counted the Daphnias in each half of every vessel, and then transposed the coverings, so that the half which had been covered was left exposed, and vice versa. I also changed the Daphnias from time to time.

Here, then, in each case the Daphnias had a choice between two kinds of light. It seemed to me that this would be a crucial test, because in every case the colored media act by cutting off certain rays. Thus the aurine owes its yellow color to the fact that it cuts