“March 2, 1882] 
It seems clear, therefore, that the ultra-red is to them practi- 
cally the same as darkness. 
I then so arranged the trough that the yellow fell in the middle 
of one of the divisions. The result was :— 
Number of Daphnias 
Upper edge 
Ultra-red of red, Greenish 
and yellow, and blue and Violet. Ultra-violet. 
lower red. lower ue. 
green. 
Exp.it. | 428 38 4 fo) fo) 
op. ate A) 36 5 (0) {o} 
esis cso & 39 3 fo) fo) 
25 113 12 fo) to) 
I then shut them off from all the colours excepting red, giving 
them only the option betweea red and ultra-red :— 
Red, Ultra-red. 
EX pe lemereeseal ss ee. 8 40 4 
NA el giee cea weeny U/ 3 
1 3s - 44 6 
137 13 
I then left them access to a division on the other side of the 
red, which, however, I darkened by interposing a piece of wood. 
This enabled me better to compare the ultra-red rays with a 
really dark space :— 
Dark. Red. Ultra-red. 
Xp elow nse Cy se” 4 43 3 
rth WE Eh sree} 45 2 
7 88 5 
Certainly, therefore, their limits of vision at the red end of the 
spectrum seem approximately to coincide with ours. 
I then proceeded to examine their behaviour with reference to 
the other end of the spectrum. 
Ultra-violet. Dark 
58 2 
286 14 
Not satisfied with this I tried to test it in another way. 
I then shut them off from all the rays except the blue, violet, 
and ultra-violet. The resul: was as follows :— 
Number of Daphnias 
Ulira-violet. Violet. Blue. Dark. 
JOP 9S iG. SERB» cermin 9 38 2 
rhe Sone deerme 6 38 2 
oss ° 2 46 2 
5 17 122 6 
I then gave them only the option of ultra-violet, violet, and 
darkness :— 
Ultra-violet. Violet. Dark. 
Exp, I a 48 4 
ae ze 6 48 6 
yp 3 12 47 I 
» 4. 15 42 3 
» 5 4 53 3 
45 238 17 
I then tried ultra-violet and dark. The width of the violet 
was 2 inches; and I divided the ultra-violet portion again into 
divisions each of 2 inches, which we may call ultra-violet, 
further ultra-violet, and still further ultra-violet. The results 
were :— 
Number of Daphnias 
Still further Further Ultra- 
ultra-violet. ultra-violet. violet. Dark. 
Exp. I. fo) 2 2 
aes fe} 5 52 3 
“pee fe) 6 50 4 
4s ° 4 53 3 
a Se ie) 4 54 2 
° 25 261 14 
May 18.—I then again tried them with the ultra-violet rays, 
NATURE 
423 
using three divisions, namely, further ultra-violet, ultra-violet, 
and dark. The numbers were as follows, viz. under the 
Further . 
ultra-violet, Ultra-violet. Dark. 
Dy gor fem sail alas) 50 4 
PR Die dees! fe, SS) 55 2 
9 105 6 
To my eye there was no perceptible difference between the 
further ultra-violet and the ultra-violet portion ; but slightly 
undiffused light reached the two extreme divisions. It may be 
asked why the still further ultra-violet division should have been 
entirely deserted, while in each case two or three Daphnias were 
in the darkened one. This, I doubt not, was due to the fact that 
the darkened division being next to the ultra-violet, one or two 
in each case straggled into it. 
Ithen placed over the ultra-violet division a glass cell containing 
a layer of sulphate of quinine about jinch in depth, and over the 
further ultra violet a similar cell with water. I had expected 
that the great majority would have collected under the water-cell. 
The numbers, however, were :— 
Ultra-violet with 
cell containing 
Further ultra-violet 
with cell containing 
water, sulphate of quinine. 
Exp. 1. 50 
» 2 4 54 
” 3: II 49 
ny 4s 4 56 
27 209 
The reason of this, however, seemed evident as soon as I tried 
the experiment ; because thoagh the sulphate of quinine stops 
the ultra-violet rays, it turns them into blue light, and, to our 
eyes at least, actually increases the brilliance. 
I then took a cell in which I placed a layer of 5 per cent. 
solution of chromate of potash less than an eighth of an inch in 
depth, which, though almost colourless to our eyes, completely 
cut off the ultra-violet rays. Ithen 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 :— 
Side of the ultra- 
violet covered Side un- 
with chromate of _ covered. Dark. 
potash. 
EXPplia vg) spe 5 55 ° 
I now covered up the other side. 
Be ae iene 57 ° 
Again covered up the same side as at first. 
sis Shee eed: 56 fo) 
Again covered up the other side. 
in qian ae 57 fo} 
May 19.—Again the same arrangement. I reduced the chro- 
mate of potash to a mere film, which, 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 experi- 
ment I reversed the position of the two cells. The numbers 
were :— 
Under the film of Under the 
chromate of potash, water, 
Exp. I. 43 52 
mo dl 56 
ee Shee ees LO 50 
oe 7 53 
29 211 
Evidently 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 film of a 5 per cent. solution 
of chromate of potash only § inch in thickness, which cuts off the 
ultra-violet rays, though absolutely transparent to our eyes, is by 
no means so to the Daphnias. 
I then again returned to the sulphate of quinine; but instead 
of placing it close to the water, I suspended it at a height of 3 
