126 PROCEEDINGS OF SOCIETIES. 



described in Mr. Mosley's paper, read to this Section on 21st Janu- 

 ary, 1861, published in the 'Quarterly Journal of Microscopical 

 Science,' vol. i, new series, p. 143, and is found to answer better 

 than any other method yet made known. 



Professor Williamson presented fourteen specimens of dredgings, 

 supposed to be from the mouths of the Ganges. 



A communication from Mr. A. G. Latham was read, upon the 

 subject named for the evening's discussion, " On the cause of the 

 metallic lustre on the wings of the Lepidoptera, both diurnal and 

 nocturnal." 



Mr. Latham believes that the metallic lustre may be simply re- 

 ferred to the presence of a pigment in the substance of the wing, in 

 some cases light-absorbing, and in others light- reflecting ; all the 

 scales seem equally adapted for reflecting the prismatic colours, 

 consisting of three distinct membranous films, covered with minute 

 irregularities. Mr. Latham sent to be exhibited a number of slides 

 for illustration. 



A communication was read from Mr. Dancer, in which he referred 

 to a paper, read by Sir D. Brewster at the last meeting of the British 

 Association, containing the following remarks by Professor Dove : 



"In every case where a surface appeared lustrous there was 

 always a transparent or transparent-reflecting stratum of much 

 intensity, through which we see another body ; it is therefore ex- 

 ternally-reflected light in combination with internally-reflected or 

 dispersed light, whose combined action produced the idea of lustre. 

 * * This efi'ect we see produced when many watch-glasses are 

 placed in a heap, or when a plate of transparent mica or talc, 

 when heated red-hot, is separated into multitudes of thin layers, 

 each of which, of inconceivable thinness, is found to be highly 

 transparent, while the entire plate assumes the lustre of a plate of 

 silver." 



Mr. Dancer sent for exhibition several pieces of talc, which in 

 places, by the action of the blowpipe, had been heated to redness ; 

 the films were thereby separated, and the raised or blistei'ed portion 

 gave a metallic lustre like silver. 



Mr. W. C. Unwin believed that the metallic lustre was due 

 not to pigment but to the reflection of light from internal surfaces 

 of the scales through the transparent outer layer. The light 

 so reflected appeared to be modified in two Mays — by the ribs 

 or striae ; it was dispersed by them so that the scales were lustrous at 

 various angles, and it was also in some cases coloured by interference 

 caused by them. Iridescence appeared to be also produced in 

 some scales by the thinness of the lamiuse through which the 

 light was refracted causing interference. Mr. Unwin exhibited 

 a number of specimens to illustrate his arguments. 



Mr. Dale referred to beautifully coloured films which arise upon 

 various chemical solutions, the metallic brilliancy of which may 

 arise from similar causes. 



Mr. Sidebotham observed that the metallic appearance was not 



I 



