60 The Angular Aperture of Objectives. 



Now, the semi-cylinder of common crown glass is just the thing 

 to produce that case, and admit of no other. The plane surface of 

 the semi-cylinder controls. If the balsam is like the glass as to 

 index no deviation takes place between them. If thin with tur- 

 pentine, considerable deviation, but the reading on the cylindrical 

 surface the same. If water, the angle will appear the same. This 

 is according to actual trials when the apparatus was first mounted. 



Therefore, and also, the semi-cylinder would correctly represent 

 the balsam angle whatever the medium between objective front 

 and cover — only if (taking the case of the -1-inch objective named 

 herein) the medium be so rare as to give to glass an angle of 

 interior total reflexion less than the objective's angle in balsam. 

 Thus, with air between objective front and cover, only 80"^, or 

 closely to that, was obtained, although in balsam tank the objective 

 showed 110^. 



I may as well add, that if anyone in England thinks proper to 

 construct an objective to gain this angle in balsam, of course my 

 remark as to comparative limit of angle would not apply. I have 

 pointed out how in the case of the four-system objective (' Monthly 

 Microscopical Journal,' March, 1872). 



The 110^ i of the tank list is such an objective ('Monthly 

 Microscopical Journal,' No. XLV., p. 106), but it was not built for 

 the maximum in balsam by a long way. The same objective with- 

 out change of construction is the identical one-fifth tested in 

 Washington by Dr. J. J. Woodward and associated gentlemen, and 

 declared to have " over 100'^ of balsam angle." 



The means of making an objective of three systems to have as 

 much angle, I have suggested at least in the same paper, and 

 since produced, as stated in my note in your last issue. 



In conclusion, I have to remark, that as all my measurements 

 of angle given in the tank list, and since, have been always made 

 with adjustments for cover, giving good definition, therefore, Mr. 

 Wenham has not yet answered. 



In 'Monthly Microscopical Journal,' No. LI., p. 123, Mr. 

 Wenham says, — " Mr. Tolles, in admitting that he closes the lenses 

 within the position of proper definition, gives us the key to his 

 fallacy." This " admitting " I pointedly deny, and call on Mr. 

 Wenham to point out where he finds such admission. 



Boston, June 20th, 1873. 



