282 



PROCEEDINGS OF SOCIETIES. 



The diagram, Fig. 1, is his own (except the dotted lines), showing 

 118^ to be the utmost possible for the lens, on the assumption that 

 the whole diameter, • 043", is used, and that all the rays cross at the 

 focal distance of •013" in air. The dotted lines added by me cross 

 at '018" distance from the face of the lens, and show an angle of 



Fig. 1. 



Fig. 2. 



100^. Were the whole exposed front surface used, this would be the 

 balsam, i.e. glass angle of the objective, provided the cover thickness 

 is given correctly. 



Fig. 2 shows the same diameter, "OiS", and the same focus 

 through covering glass, = • 018", and a used diameter of • 036" ; thus 

 twice the focal distance ( — ' 036") represents the used diameter 

 giving a right-angled triangle with 90° at the focus. 



His own later " utilized diameter " makes the balsam angle some- 

 what less, closely to 88°, but leaves the argument conclusive against 

 his assertions of small angle in the objective. 



By my own measurement the working distance of the objective 

 through glass cover is less than •018", and accordingly the balsam 

 angle I claimed and claim is more than his data afford, but either 

 exceed the equivalent (82°) of 180° in air. 



Yours respectfully, 



K. B. TOLLES. 



PROCEEDINGS OF SOCIETIES. 



EoYAL Microscopical Society. 



King's College, May 3, 1876. 



H. C. Sorby, Esq., F.R.S., President, in the chair. 



The minutes of the preceding meeting were read and confirmed. 



A list of donations to the Society was read by the Secretary, and 

 the thanks of the meeting were voted to the donors. 



Mr. Charles Brooke said he had a very pleasing duty to perform 

 before proceeding to the business of the evening, and that was to 

 propose that the best thanks of the Fellows of the Society be given 

 to their President, Mr. Sorby, for the soiree which be gave to them 



