320 PROCEEDINGS Or THE SOCIETY. 



which it was proposed to make in the classification of the organisms, 

 and in the book before them it would be found that the altered method 

 had been followed. 



Mr. Crisp exhibited a Microscope which had been made to order 

 for the purpose of examining large specimens of minerals (supra, 

 p. 274). 



Mr. J. Mayall, jun., thought it would be unfair to criticize an instru- 

 ment of that sort except as to its adaptation for the purpose for which 

 it was designed. This one, notwithstanding its unusual appearance, 

 might answer its purpose very well, and he thought if a person had a 

 block of granite or quartz to examine, he would hardly like to use a 

 valuable Microscope of ordinary construction for the purpose. 



Mr. T. F. Smith, exhibited a number of photomicrographs of Podiira 

 scales and diatoms taken with one of Zeiss's apochromatic objectives, 

 the former showing secondary markings not previously described, and 

 some illustrating the diiference of appearance presented by the same 

 object with different corrections of the objective ; the peculiarities 

 presented were further illustrated by drawings upon the blackboard. 



Professor Stewart said he should be glad to know what was the result 

 of Mr. Smith's final examination as to the general meaning of the entire 

 structure of the Podura scale. In a specimen which he saw exhibited at 

 the Quekett Club, the turned-over edge of the scale was clearly shown, 

 and the interrogation marks appeared to be upstanding processes from 

 the surface of the scale (drawn on the board). What did Mr. Smith 

 think was the real structure of the scale, the upper surface being smooth 

 and the lower apparently bearing these projecting markings ? 



Mr. Smith said he had at present no definite idea of the real struc- 

 ture, because he found that when the scale was examined in media of 

 high refractive index, the whole appearance was altered. His impres- 

 sion was, however, that these markings lay between two membranes, 

 one corrugated and one plain, the latter being at least as thin as 

 1/140000 in. In butterfly scales the existence of a second membrane 

 could be sometimes shown, but in others, and in the Podura scale, it was 

 optically invisible. 



The President inquired how, if the markings were between two 

 membranes, the appearance of their projection could be accounted for ? 



Mr. Smith said he thought that there was a fine membrane spread 

 over the surface of the scale, and that the markings extended between 

 the two. He regarded the projections as being real, although they did 

 not really stand out from the outer surface of the scale in consequence 

 of being included within a fine membrane, which was in itself too 

 delicate to be optically visible. 



Professor Stewart suggested that it was rather a dangerous proceed- 

 ing to assume the existence of a membrane which they could neither S(.e 

 nor demonstrate by any means whatever. 



' Mr. J. Mayall, jun., said that Mr. Smith had come forward re- 

 peatedly, both there and in other places, attempting to determine the 

 structure of objects of this class from the appearances presented. He 

 had himself, had perhaps as much experience in these matters as most 

 persons, and he could only say that to attempt to interpret such struc- 



