374 PROCEEDINGS OF THE SOCIETY. 



totally reflected, aud thus the valve illuminated itself. On removing 

 the eye-piece when the object was so resolved, the diffraction spectrum 

 and the reflected illuminating beam would be visible at the extreme 

 edge of the back lens, on opposite sides ; showing, as he had pointed 

 out on a previous occasion, that although it was a dry object, the full 

 aperture of the wide-angled immersion objective beyond the equiva- 

 lent of 180° in air had been utilized, which could never be the case 

 with transmitted light. In other words, the resolution in this case is 

 exclusively effected by what have been contended to be " non-image- 

 forming rays," or rays in excess of 180° in air. If an opaque object, 

 such as the elytron of a beetle, were examined, it would of course 

 reflect the light brilliantly under either of the conditions he had 

 named ; but with a transparent object, such as A. pellucida, this would 

 not be so. 



Mr. Powell said that the diatom was in fact invisible, as Mr. 

 Stephenson had suggested, when the bright annulus was excluded. 



Mr. T. Charters White said he had often thought that for the 

 study of insect anatomy it would be a great advantage if they could 

 get some kind of reagent which would render the chitinous envelope 

 transparent ; and he was at first rejoiced to hear that carbolic acid 

 would produce this result. He was, however, disappointed to find in 

 jiractice that whilst it rendered the chitinous envelope transjiarent, it 

 also made the internal organs equally so. This was so serious an 

 objection that he had resolved to try and work out the subject during 

 the summer, with the idea of finding something else which might not 

 be open to the same objection. He mentioned the subject now, in the 

 hope that other Fellows would co-operate. 



Dr. Matthews said he had tried carbolic acid, but the experiments 

 which he had made were not so successful as to induce him to use it 

 again. He produced a slide of a whole spider which had been prepared 

 in carbolic acid. When taken out it was nearly transparent, and appa- 

 rently in a very favourable condition for mounting ; but it would be 

 seen that the whole of the abdomen had collapsed, a circumstance 

 which he coiild only account for by supposing that the balsam and the 

 carbolic acid had entirely changed j)laces. This specimen was 

 mounted in undiluted balsam, and he had tried since to mount others 

 in various solutions of balsam, in benzole, ether, and chloroform, and 

 had also tried common pine resin in copaiba, but all had failed in 

 consequence of this aj^parent endosmose ; and he was quite at a loss 

 what to try next. He should mention that when first mounted the 

 spider was perfect, and all that could be wished, but the next morning 

 it was found in its present condition. 



Mr. Stewart said he was without special experience in the matter, 

 but should like to know whether the object was made clean by the 

 simple extraction of the watery contents, or whether there was any 

 kind of bleaching action about it ? 



Dr. Matthews said he had not yet established the fact that it had a 

 bleaching effect, but he susijected that such Avas the case. He regretted 

 very much that a jirocess which seemed to promise so well at first, 



