1886.] On the Polarisation of Light by Iceland Spar. 



173 



body of lepidopterous larvae which adopt the methods of protective 

 resemblance. Furthermore, it is very probable, as suggested by 

 Professor Meldola, that the colour of the environment will prove to 

 act as one of the determining causes of the larval colours ultimately 

 assumed by the individuals of dimorphic species (which are generally 

 green and brown in lepidopterous larvae). To show in what a light 

 this colour-relation appears to Dr. August Weismann (whose essay 

 upon " The Markings of Caterpillars" first induced me to work at 

 these organisms), I quote the following sentences from a letter I 

 received from him after sending him my paper in the " Entomological 

 Society's Transactions," Part I, April, 1884, in which this subject is 

 alluded to : — 



" Dagegen verstehe ich nicht ganz, wie sie sich den 'phytophagic 

 character of the ground-colour' entstanden denken. Ich habe 

 augenblicklich mein Buch nicht zur Hand u. kann deshalb die Note 

 von Meldola nicht nachsehen, erinnere mich auch derselben nicht. 

 Sie scheinen zu glauben, dass die Nahrung der Raupe bis zu einem 

 gewissen Grad ihre Farbe direht hervorrufe. Ich ware sehr begierig, 

 einen Beweis dafur kennen zu lernen. Ich kann mir nicht denken, 

 wie dies moglich sein, solle jedoch weiche ich den TJiatsachen ! Ich bin 

 begierig, zu erfahren, ob Sie solche inzwischen gefunden haben." 



I venture to hope that the facts spoken of by Professor Weismann 

 are now satisfactorily demonstrated, not as proving the former theory 

 to which he alludes, that the food itself causes the change of colour 

 after it has been eaten by the larva, but as proving the existence of 

 the more subtle form of influence described in the present and in my 

 last paper. At the same time I must express my sense of the great 

 extent to which I am indebted to Professor Meldola, to whom we owe 

 the former theory, for without the most suggestive editorial notes to 

 his translation of Weismann's work, and the experiments undertaken 

 by Mr. Boscher at his request, it is most improbable that the present 

 investigation would ever have been begun. 



III. "On the Polarisation of Light by Reflection from the 

 Surface of a Crystal of Iceland Spar." By Sir John 

 Coxroy, Bart., M.A., of Keble College, Oxford. Communi- 

 cated by Professor G. G. Stokes, P.R.S. Received 

 January 27, 1886. 



(Plate 2.) 



In the year 1819 Sir David Brewster communicated to tile Royal 

 Society ("Phil. Trans.," 1819, p. 145) an account of some experiments 

 he had made on the polarisation of light by reflection from the surface 



