PROCEEDINGS OF THE SOCIETY. 1089 



not do SO, neither did that of Messrs. Powell and Lealand — indeed they 

 did not pretend that it did. He had, therefore, no hesitation in saying 

 that any stereoscopic etfect in such an instrument must be formed 

 entirely in the imagination of the observer, just as a seal might be seen 

 under certain conditions, and might be imagined to be either a sunk 

 impression or a raised cameo ; for when one had got a conception 

 of solidity, this mental conception might easily be carried on. 



He had found the binocular to be essential to a knowledge of solid 

 form, and as he had often expressed that opinion, a number of foreign 

 microscopists had from time to time come to him upon the subject. 

 One day Professor Haeckol, amongst others, seemed, as most conti- 

 nental observers used to be, rather doubtful as to the value of the 

 binocular. He showed him at first some Polycistina, which were of 

 course familiar to him as solid objects, and he looked at them and 

 said he did not see them ditferently from usual. Then he showed 

 him an object which he had not seen before — it was a piece of the 

 wing of a small moth, which had a peculiar arrangement of the scales. 

 Drawing out the prism, he asked the professor to look at the object 

 and to adjust the focus so as to get a good middle distance, and then 

 whilst he was looking at it the prism was suddenly replaced. He 

 quite started at the result, for he then saw the undulations as if they 

 were a solid raised surface, and admitted at once the true character of 

 the stereoscopic effect produced. The mere fact that altering the 

 caps as described by Mr. Lettsom was stated to be competent to 

 change the image from stereoscopic to pseudoscopic satisfied him at 

 once that there was no true stereoscopic effect produced. 



Mr. Crisp referred to several communications upon the subject 

 of wax cells, American correspondents more particularly bcin*' of 

 opinion that Professor Hamilton Smith had given tliem up too hastily, 

 and tliat the sweating comi)hi,ined of came from the use of cements 

 containing turpentine, &c., and other causes apart from the wax (see 

 p. 1039). 



Mr. Shadbolt's Memorandum on Apertures "exceeding 180° in 

 Air " was read as follows : — 



" It was with c(Jiisidcrable regret that I found a short article in the 

 October nunibur <»tthe 'Journal' (i).875), entitled 'Apertures exceeding 

 180 in Air,' especially a.s it was not followed by any editorial com- 

 ment of warning against the errors of both theory and fact involved 

 therein. 



In the article in question I find the following sentence, viz. : — 

 ' The whole confusion has arisen from not getting beyond the simplo 

 and obvious fact, about wliich there can bo no dispute, that a </ry lens 

 cannot have an aperture of more than 18U\' To anyone who cannot 

 grasp this 'obvious hut,' 1 doubt th.it any explanation of optical plie- 

 noHKiia would bt! inttdiigible. The sUitenient is true, certainly ; but 

 it is also misleading, because it is only a part of the truth. The whole 



