Lae 
May 28, 1885] 
quite different notation is employed—is simply infuriating! I 
would urge upon Prof. Pearson that he has now an unrivalled 
opportunity of fixing in the language of English (and perhaps 
foreign) mathematicians a really serviceable and significant 
system of notation. 
The double-suffix notation for strain and stress, which is deve- 
-loped to perfection in St. Venant’s French translation of Clebsch, 
has many advantages, but seems to be too cumbrous for English 
taste. Nothing perhaps could be more unmeaning than Thomson 
_and Tait’s notation for ‘‘ stresses,” independent as it is of all 
reference to the strain-symbols. Still I must confess (in common, 
I dare say, with most men who have derived their first inspira- 
tions from that mathematical epic) that it has secured too firm a 
place in my mental machinery to be lightly cast out, even in 
favour of a better. W. J. [BBETSON 
Cambridge, May 12 
The Colours of Arctic and Alpine Animals 
Mr. R. MeLpo.a has maintained, in NATURE, vol. xxni. 
Pp. 505, the idea that the white colour of some animals, 
Arctic mammals and birds, must be ascribed to the absorbent 
and radiating power of the same colorations in relation to the 
rays of the sun. He maintains also that to a similar cause we 
owe the seasonal polychromism of several mammals and birds of 
the Alps, and what would be for these animals a partial return 
to the characters of the Glacial epoch. 
By an analogous theory the author explains the contrary 
phenomenon that is observed in many insects—that is, the 
darkening of the coloration, and he speaks principally on this 
point of the Lepidoptera. 
Now I begto make the following observations, and to indicate 
the following facts :— 
(1) That a seasonal mutation of colour is observable in many 
Mammals, now more, now less distinctly, and generally 
it concurs with the change of coat. Also not seldom in 
mammals strictly belonging to the Alps, as, for example, in the 
Rupicapra europea, and in the Capra tbex, the colour changes 
very little in the summer and in the winter, although the 
length, the thickness, and also the coarseness of the hairs were 
very different. In other cases, as, for example, in the Cervus 
mandarinus,* the coat is, in summer, light reddish yellow, with 
many round white spots, while in winter it is dark brown, and 
the round spots are less numerous and are light brown. 
(2) As to the insects, it is observed that in Coleoptera the 
colours of the Alpine species are brighter than those of the 
warmer plains, as in the genera of Carabus, Plerostichus, &c. 
Iniseveral species of Harpalus, Amara, Cicindelis, &c., the 
individuals that we find at the greatest elevations of the Alps 
have often lighter colours. 
(3) A darker colour and sometimes a whole melanism is ob- 
served in general in the insects of the deserts—for example, in 
that of Sahara. On the contrary, the mammals of these coun- 
tries present in general a very light colour. It seems to: me 
that this fact cannot be explained by the theory of radiation. 
(4) A very remarkable melanism is also observed in several 
mammals, the Reptilia and Coleoptera that are in little islands, 
or upon rocks in the warmest regions, for example the Z. mzvadlis, 
&e., Cicindela campestris, in the island of St. Peter in Sardinia.? 
(5) In the reptiles and in the Alpine amphibia we sometimes 
meet with some cases of darkening, but the cases of aremarkable 
brightening are not very rare, as, for example, in the tadfoles of 
Rana muta. 
(6) A sensible difference is observed in the coloration between 
the Arctic birds and the Antarctic. In these last black is much 
more abundant. 
Indeed, Australia, New Zealand, &c., are countries known 
for a remarkable darkening in the colours of many sorts of 
animals. 
In the Carnivora, which are the mammals that chiefly pre- 
sent seasonal polychromism and white colour, is observed a 
_tendency to this colour in several forms that, however, do not 
live either in Polar regions or in very cold places. As to this 
fact the colour of the genera Zoril/a, Meles, &c., and also the 
very curious Az/urus melanoleucus of Thibet,? should be ob- 
served. 
* Milne-Edwards, ‘‘ Recherches pour servir a I’Histoire Naturelle des 
_Mammiferes,” tay. 22, 22a. Paris: Masson. 
= Siconsulti L. Camerano, ‘‘ Richercheinterno alla Distribuzione dei Colcri 
nel Regno animale.” Mem. R. Accad. Scienze di Torino. 
3 Milne-Edwards. 
NATURE 
_ (vol. xxxii. ‘p. 30). 
77 
_ The causes, I would say in conclusion, that intervene to 
modify the colour of animals, are very complicated ; climate 
has amongst these a certain importance, but it does not seem to 
me that, although it be very attractive, Mr. Meldola’s theory of 
radiation is sufficient. LORENZO CAMERANO 
Zoological Museum of Turin 
On Certain Stages of Ocular After-Images 
In a short note in the Phz/. Mag., 1872, vol. xliii. p. 343, 
Prof. C. A. Young has recorded a curious instance of ‘‘after- 
image,” which seems to me to be of the same order as that 
observed by Mr. Shelford Bidwell, and recorded in NATURE, 
I quote from Prof. Young’s note, which is 
named ‘‘ Note on Recurrent Vision,” a few lines, which will 
show what his observation was :— 
“Tn the course of some experiments with a new double-plate 
Holtz machine belonging to the College (Dartmouth, America), 
I have come upon a very curious phenomenon, which I do not 
remember ever to have seen noticed. The machine gives easily 
intense Leyden-jar sparks from 7 to 9 inches in length, and 
of most dazzling brilliance, at the rate of seventy a minute. 
When, in a darkened room, the eye is screened from the direct 
light of the spark, the illumination produced is sufficient to 
render everything in the apartment perfectly visible ; and, what 
is remarkable, every conspicuous object is seen fwzce at least, 
with an interval of a trifle less than a quarter of a second—the 
first time vividly, the second time faintly ; often it is seen a third, 
and sometimes (but only with great difficulty) even a fourth 
time.” 
Prof. Young shows that it is a subjective phenomenon, and 
measures the interval between the first and second seeing of an 
object, giving as the mean of twelve experiments the interval 
0°22 second for the case of his own eyes, and 0°24 second for 
that of another observer. : 
Five or six years ago I observed another instance of what I 
believe to be the same kind of ‘‘after-image,” though at first I 
was inclined, being engaged upon experiments with a view to 
finding the cause of certain ocular ‘‘ghosts’”’ due to multiple 
reflection inside the eye (Proc. Roy. Soc., No. 223, 1883), to 
ascribe it to a different cause. It was seen in a room lighted 
only by the bright glow of coals in the grate. Whenever the 
eyes were suddenly flashed across the fireplace, and then fixed 
on some object 50° or 60° from it, there appeared a faint blue 
light, which seemed to flash from the object to the glow. This 
phenomenon was much more strongly marked at some times 
than others, and varied withsome cause which I never further 
investigated. Later I came upon another instance of the same 
thing ; and as this is the easiest to reproduce, and one by which 
one may best study the phenomena, I will describe it. 
Let a match or a splinter of wood be made to glow, as for 
testing oxygen, and let it be observed in a dark room; the eyes 
should be fixed, and the glowing match moved about. I found 
that for purposes of rough measurement a most convenient curve 
of motion is a figure of 8 on its side in a vertical plane (). 
Also it is convenient to keep the period of the movement the 
same, and to vary the size of the curve if change of velocity is 
required. There are difficulties to be overcome in regulating 
the brilliancy of the light (Mr. Bidwell has pointed out the 
necessity of a certain degree of brilliancy in the case of the 
vacuum tubes), if a systematic investigation were undertaken ; 
a glowing match becomes brighter the quicker the movement ; 
the reverse is the case with a platinum wire carrying a strong 
current of electricity ; and asmall incandescent lamp is objection- 
able on account of reflection from its glass case. 
I shall consider the ‘‘after-images” of the glowing-point as 
forming a trail, in which all the changes are set out at the same 
moment, and proceed to describe the trail for two cases. I 
should state that following descriptions refer to the trails as seen 
by me 27 ¢he evening ; for there are very considerable variations 
in the phenomena according as the eye is likely to be wearied 
or fresh. I may also repeat Mr. Bidwell’s caution that it is by 
no means certain that a person new to the subject will at first be 
able to see the appearances described. 
I arrange a metronome beating seconds, and move the glow- 
ing-point so as to describe the curve completely in two seconds. 
First, let the figure of eight be only as large as can be got into 
a rectangle 3 inches by 14. In this case there comes after the 
glowing-point a dark interval in the trail, about an inch long ; 
then a distinct blue-green ghost, about the same size as the 
