908 PKOCEEDINGS OF THE SOCIETY. 



rection-adjustment to homogeneous-immersions that he now agreed 

 with Dr. Zeiss that those objectives should be supplied either with 

 or without adjustment ; accordingly, in future, both kinds would be 

 supplied. He gave this on the authority of Dr. Zeiss. Prof. Abbe 

 must therefore be regarded as partially, at least, opposed to Dr. Dippel's 

 views. He might add that, among the opticians who were in favour 

 of mounting homogeneous-immersions with correction-adjustment, 

 were Powell and Lealand, Eoss, Schroeder, Spencer, and Tolles. 



Mr. Crisp said that Dr. Dippel did not dispute that a somewhat 

 higher degree of accuracy might be obtained with the correction- 

 collar, but it was only with known objects, such as the Abbe test- 

 plate, and with the closest examination. With unknown objects, 

 however, he considered it was utterly impossible to determine the 

 position of best correction, and the correction-collar had, therefore, in 

 those cases, no advantage to compensate for its disadvantages, and 

 should not be used by histologists at any rate. 



Mr. Crouch said that, in making the rough adjustments for students' 

 Microscopes, they had to bear in mind the purposes for which they 

 were likely to be used. In ordinary cases they would have to provide 

 for histological sections, and this was not an easy matter, being quite 

 unlike the case of a Podura scale, where they could adjust properly, 

 because they had a surface to focus upon. He found the best average 

 results were obtained from an objective when it was adjusted for an 

 average thickness of cover-glass. He had known cases in which 

 objectives with correction-collars had been condemned and returned 

 to him, because it was said that good definition could not be got, the 

 same objective being pronounced satisfactory after being remounted 

 in a fixed setting. 



Dr. Edmunds thought that adequate weight had not been given to 

 the difficulty of ascertaining what was the true image of a complex 

 histological section when viewed under a high-power objective. The 

 real question was whether that image could be most certainly fixed 

 upon by means of a lens furnished with a correction-collar, ne- 

 cessitating its adjustment for each object. Microscopists who devoted 

 themselves to the resolution of Amphipleura pellucida, or of Nobert's 

 lines, often knew nothing of the difficulty of interpreting the structure 

 of muscular fibre and other more complex histological objects. The 

 Podura-scale, though used as a test-object for histological lenses by 

 their makers, yet was an object whose structure had never yet been 

 interpreted in any way which commanded general assent. Take, 

 again, Pleurosigma formosum in balsam, which presented different 

 appearances with every touch oi the correction-collar and with every 

 variation in focal distance, so that it could not be interpreted with 

 certainty ; were a skilled microscopist now to see this object for the 

 first time, a correction-collar upon his homogeneous lens would only 

 add to his difficulties in fixing upon what was its true image. With 

 a water-lens and varying thickness of cover-glass, the case was 

 different, and the correction-collar was indispensable. While, there- 

 fore, the correction- collar might be theoretically an advantage, he 

 thought that in practice it would be a disadvantage, and that, instead 



