SOTKNTIFIO NOTES ANP OBSERVATIONS, 205 



Thinking over the change of colour of Thecla rubi l>y wetting, may 

 not the following be an explanation ? There seems to be green and 

 red pigment in the scales, the green being superficial. When white 

 light strikes these gi'een granules, a great part is reflected from their 

 external surface as white light, which, however, carries with it some 

 green that has been reflected from tlie posterior surface after penetra- 

 tion. The white light penetrating deeper meets with the red granules, 

 but owing to the dark back-ground most of it is absorbed. What little 

 red is returned mixes with its complementary green and forms white 

 light, which slightl}' diminislies the intensity of the I'cflected gTeen. 

 When light is transmitted there is much more red light, part of which 

 forms white light by uniting with the green, and the remainder meets 

 our eyes as red. When water is added, the amount of superficial 

 reflected light is vei'y much diminished, because the light passes 

 through water instead of air (a dense medium instead of a rare one), 

 consetpiently more red light, proportionately, is reflected, part of which 

 combines with the weaker gi-een to form white light, and the rest 

 appears to us as red. Were it connected with the interference of light 

 from the strife of the scales, as Mr. Tutt suggests, should not the C(dour 

 vary with position, as in Thecla qnerctis ? I tried to verify this ex- 

 planation by experiment. I painted some carmine on one glass slide, 

 and a smaller patch of bluish green on another, and then i)laced the 

 latter on the former. By reflected light the colour was green, by 

 transmitted reddish-brown. Looking at them by light passing through 

 water, where the green covered the red it was no longer visible as such, 

 but looked reddish-brown. The difficulty is in getting right shades of 

 colour, but carmine and yellowish-green, to which I added some Italian 

 blue, when superimposed, produced a brown not unlike that of T. rubi. 



Dr. Freer does not quite seem to see that I merely laid stress on 

 correct definitions of the terms — pigment, white, black, etc. — to show 

 that there is a material something in the scales of moths which is acted 

 iipon by light and is a factor in the production of colour, and that it is 

 reasonable to suppose that this material something may be altered by 

 cliemical agents, etc., so that light may affect it differently. I am not 

 (piarrelling witli the terms we daily use. We must and always shall sj^eak 

 of black pigment, red pigment, isolation of pigment, ikc. ; but I wished to 

 suggest that we should not forget the limitations of their exact meanings, 

 as it appears to me, that only so can we realise how chemical 

 agents, heredity, etc., may act in bringing about and perpetuating 

 clianges of colour. I agree with Mr. Burrows in thinking pigment 

 the usual cause of colour in Lepidoptera ; on the other hand, J)r. Freer 

 seems to think their colours mostly those of light — and among others, 

 brings forward in proof our T. querem. All the dark brown of this 

 insect seems certainly due to pigment granules in the scales, but I also 

 notice that the more superficial scales which give the purple colour 

 (undoubtedly by dispersion of liglit, as Dr. Freer says), are also coloured 

 pale yellow or yellowish -brown. Is it not probable that this yellow 

 pigment explains the purple colour ? Wlien wliite liglit is dispersed 

 by the stria3 of the scales, may not the yellow liglit combine with the 

 dispersed blue — its complementary colour— and form white light, 

 whilst the remaining waves of red and violet mix to form the purple 

 we see. How else can we explain the i)urple ? If this be the case, 

 does not pigment, even in T. qnercus, play by far the largest part in the 



