466 Mr. Voulton' s fnrthei' experiments upon 



pigment, or dismiss it and become green, or in some 

 cases can alter the constitution of the former so as to 

 produce a variety of tints, each of which is more or less 

 appropriate to some form of natural environment. 



The use of coloured screens obviously alters the case 

 entirely, for the larval surface is thus exposed to far more 

 concentrated rays from certain parts of the spectrum. 



The screens employed were kindly described for me by 

 Sir John Conroy. I quote his determinations below: — 



Red gelatine. — "Transmitted light. Eed. The ab- 

 sorption begins abruptly on the less refrangible side of D 

 at about a wave-length of 604, and extends through the 

 remainder of the spectrum." — Keceptacle LXXII. (see 

 G, Appendix). 



Although the light was thus a very pure red, the 

 larvfe of V. io formed distinct green pupae when exposed 

 to it upon a reflecting background. In this concentrated 

 form, red rays have the power only possessed by yellow 

 and orange under more normal conditions. 



Red glasa. — " Transmitted light. Red and some yellow, 

 together with a little green and a trace of blue. Light of 

 wave-lengths between about 589 (D) and 559 strongly 

 absorbed."— Receptacles LXXL, LXXII. (at one time), 

 LXXV. 



Green pupae of V. io were similarly formed beneath 

 this screen, and much lighter pupae of P. brassica than 

 those usually produced by a red background. 



Yellow glass. — "Transmitted light. Red, yellow, and 

 green. Slight general absorption of the red, yellow and 

 green, and strong absorption of all the blue rays." — 

 —Receptacles LXXVIII. to LXXXL, and LXXVI.^ 



Green pupae of V. io were similarly formed, and inter- 

 mediate or light pupae of P. hmssiccB. When the back- 

 ground reflected hardly any of the transmitted light, the 

 pupjT3 of the latter were darker. 



Green gelatine. — "Transmitted light. Red, yellow, 

 and green. Some general absorption through red, 

 yellow and green, and strong absorption of rays of 

 wave-length greater than 462." — Receptacle LXXIII. 



Green pupae of V. io were similarly formed. 



Green glass.—'' Transmitted light. Some red, yellow, 

 and green. Light of wave-length greater than 605 and 

 less than 501 strongly absorbed. The rays between 605 

 and 576 considerably absorbed." — Receptacles LXXXII. 

 to LXXXV. 



