906 PKOCEEDINGS OF THE SOCIETY. 



provided for the use of Lieberkiikns, &c. He had examined many 

 low-power objectives with tapering fronts, but the great majority 

 were seen at a glance to have been coned with no other purpose in 

 view than to improve their appearance. With regard to the Eoss 

 ^-inch objective of 60° air-angle, exhibited by Mr. Ingpen, of which 

 the body of the front lens was coned to an angle of 120°, it was 

 evidently not Andrew Boss's intention to cone the front to the 

 uttermost, or he would have cut it down to an angle of about 40° 

 instead of 120° : 40° in the body of the crown-glass front being 

 sufficient to transmit a pencil of 60° from air. Could Mr. Beck 

 affirm that the objective he referred to, made before 1847, was coned 

 in front so as to allow just the full aperture to be utilized? If 

 so, he (Mr. Mayall) thought Mr. Stodder's case must fall to the 

 ground. 



Mr. Beck said that the one he held in his hand was done with 

 the precise object of getting the front reduced to the smallest cone 

 possible. 



Dr. Dippel' s note " Correction- Adjustment for Homogeneous- 

 Immersion Objectives," was read by Mr. Crisp (see p. 854). 



Mr. Beck said that the paper seemed to him to be an apology for 

 being content with inferior definition rather than taking the trouble 

 to get the best that was to be obtained. He understood the course of 

 argument to be that there was so much trouble in making the adjust- 

 ment that it was better to do without it. He ventured to say that 

 there was not any one who had got over the trouble who would for a 

 moment put up with what was inferior, if he had it in his power to 

 get what was the better. He should like to hear some observations 

 on this subject from Mr. Mayall. 



Mr. Crisp said that the author of the paper did not put the matter 

 on the ground of trouble at all. What he said was that the advan- 

 tages to be derived from the adjustment-collar were so infinitesimal 

 in the case of unknown objects, that they did not compensate for the 

 disadvantages. 



Mr. Ingpen thought that even amongst those who were thoroughly 

 familiar with the use of the highest powers, very few would be found 

 capable of adjusting an objective to the same degree of accuracy as 

 the optician, and therefore any one who had a valuable objective 

 should be very careful not to disturb its correction. It was also, he 

 believed, a matter of experience that with the particular class of 

 objects referred to by Dr. Dippel, no two histological observers would 

 agree as to the best correction. Professor Abbe had met that difficulty 

 by proposing a special test-object for the purpose, and as the descrip- 

 tion of this method would, he understood, shortly appear in the 

 Journal, he need not enter further into that part of the subject, 

 except to say that the Professor's test-plate was not like the Podura 

 scale or P. angulation, but showed beyond question what the best 

 correction was, and made it possible for a person to pass a number of 

 objects under the objective, and at once determine the best correction 

 for each. 



Mr. J. Mayall, jun., said he concurred in much that Mr. Beck had 



