1^,6 



NATURE 



[October 3, 1912 



is much more important than chemical modifica- 

 tion of it. 



The author states that his researches confirm 

 in some measure a fact which seems general 

 among- animals, viz. that melanism is a sign of 

 vigour and health ; albinism, on the contrary, of 

 the enfeebling of the organism. In melanism we 

 have great abundance of pigment, increase in the 

 size of scales and sometimes in their number, 

 usually greater size, the females generally with 

 voluminous abdomen, full of eggs ; among 

 Bombyces a great increase in pilosity. In albin- 

 isnij on the other hand, we have the opposite 

 state of things; the scales also frequent'y de- 

 formed, curled, and atrophied, the wings often 

 failing to develop, the abdomen slender, with few 

 eggs or none ; in the Bombyces little pilosity. 



The author refers to Standfuss's hypothesis that 

 this enfeebling may begin in the larva, and to his 

 own experiments supporting this. Caterpillars ill- 

 nourished, having been supplied with leaves they 

 could ill bite into, produced imagines struck with 

 albinism, supporting the view that albinism is a 

 sort of anaemia of the organism. These modifica- 

 tions increase in succeeding generations subjected 

 to similar conditions, but at the end of four genera- 

 tions the caterpillars adjust themselves to the 

 leaves they found difficult, and then recover their 

 vigour, size, and habitual colour. There are 

 certain characters which exposure to abnormal 

 conditions does not seem able to modify, such 

 characters as are common to a group or genus, 

 for instance, the discoidal spot of L. quercus, and 

 the discoidal V. of Ocneria dispar. 



Dr. Pictet's valuable researches certainly cannot 

 be considered to exhaust the subject of the cause 

 of variation in the colouring of lepidoptera. In 

 the case of seasonally dimorphic species, for 

 example, experiments seem to have shown that the 

 difference in colouring is dependent more on the 

 differential life habit of the spring and summer 

 emergences than on mere exposure to environ- 

 ment or change of environment. With many 

 lepidoptera long duration in the pupal stage, such 

 as is produced by a low temperature, usually 

 causes darkness, but in A. levana cooling the 

 summer pupa for many months results in an imago 

 so much brighter than the nearly black imago 

 which comes from a pupal life in a summer tem- 

 perature of five or six days that it looks quite a 

 different species. F. Merrifield. 



THE SENSITIVENESS OF SELENIUM TO 

 LIGHT OF DIFFERENT COLOURS. 



SOME curious facts about the behaviour of 

 the selenium cell are mentioned in a recent 

 paper by A. H. Pfund.i The possibility of using 

 these cells for ordinary photometric processes has 

 often been discussed, but there are certain irregu- 

 larities in their behaviour which set a limit to their 

 usefulness. 



One obvious difficulty lies in the fact that the 

 rays of light to which the eye is most sensitive 



1 /'/!ys. Sevfen; xxxiv.. No. s, May, k,ii. 



NO. 2240, VOL. 90] 



are not necessarily those which have the most 

 effect on selenium. Pfund brings this out by re- 

 producing — what has seldom been published 

 before — a curve showing the distribution of 

 sensitiveness of this material throughout the 

 spectrum of a Xernst filament. The energy- 

 maximum in this spectrum lies far out in the 

 infra-red ; the maximum luminosity to the eye 

 (at moderate illuminations) is situated in the 

 yellow-green. The maximum sensitiveness of 

 selenium, however, is located in the red, and the 

 shape of the curve is also distinctly different from 

 the luminosity curve for the eye, so that incon- 

 sistent results would evidently be obtained if one 

 tried to compare the light from various illumin- 

 ants (having radically different energy-curves) by 

 means of the selenium cell. But it may be men- 

 tioned that, according. to a recent number of The 

 Illuminating Engineer, such cells have a possible 

 sphere for purelj' comparative measurements of 

 one and the same illuminant ; for example, they 

 have been used for studying the fluctuations in 

 daylight and the variation in illumination in the 

 course of an eclipse. 



Perhaps the most curious point brought out in 

 these researches is the dependence of the maxi- 

 mum sensitiveness of the selenium cell on the 

 intensity of the stimulus. ' With a very bright 

 light the maximum is in the red, near oy /n. 

 But as the illumination is weakened, this maxi- 

 mum becomes less definite, and ultimately, in a 

 feeble light, another maximum point, situated 

 about o"57 ;ji, appears. Ruhmer makes two kinds 

 of selenium cells. The " hard " type is most 

 sensitive to strong light, but relatively insensitive 

 to feeble stimuli, and has its maximum in the 

 red. The " soft " type, on the contrary, is most 

 sensitive to weak light, and has its maximum in 

 the green. This explains the discrepancies 

 between earlier observers, some of whom found 

 selenium to be most sensitive to yellow light, 

 while others thought that red had most effect. 



Now the interesting point to observe here is 

 that this behaviour of the selenium cell is curi- 

 ously similar to that of the eye. It is a well- 

 authenticated fact, demonstrated many years ago 

 by Sir Win. Abney, that whereas at strong 

 illuminations the maximum luminosity occurs 

 near o'5S/x in the yellow, in weak light it shifts 

 to the green, probably near o"5i-o'53fi. This 

 is known as the Purkinje effect. In very feeble 

 light the eye seems almost insensitive to red, 

 while green and blues appear an uncanny light 

 grey. Formerly this singular effect was ascribed 

 to a struggle for predominance between the 

 minute light-perceptive organs on the retina 

 known as the " rod •" and "cones." But, accord- 

 ing to a later theory, this is not necessarily so, 

 for the accentuation of the red end of the spec- 

 trum with increasing stimuli is characteristic of 

 all photo-chemical processes. 



At all events the similarity between the eye 

 and the selenium cell in this respect is very 

 suggestive. At first sight this complicated be- 

 haviour of selenium would seem unfavourable to 



