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III. — An Apparatus for Obtaining the " Balsam " Angle of any 

 Objective. By .Robert B. Tolles, U. S. America. 



Plate XV. (Lower portion). 

 [The following two communications, though intended by the 

 author to appear in different numbers of this Journal, have reached 

 the Editor simultaneously. They are therefore published in the 

 same number of the Journal. — Ed. ' M. M. J.'] 



In reference to Mr. Wenham's comments on my last communica- 

 tion there is just a word to add. 



I am not averse to the ordeal of a test of balsam angle in 

 London. I am not afflicted with Anglophobia ; never have before, 

 I am sure, been suspected of that. 



I impugn, not the verdict, nor the testing, so far as that trial 

 went. At air angle of 145° I get the same results, doubtless. 



My method was Mr. Wenham's tank-plan, as stated. I have 

 since used quite extensively a different method, putting the light 

 down through the microscope tube, the cone of light being 

 measured as emergent from the front of the objective, the object 

 also in position, in balsam, under covering glass, as in practice in 

 use of the compound microscope, and under identical circumstances, 

 and in view at option through the eye-piece. Therefore and thus 

 the extreme rays measured for angle can be identified as traversing 

 and giving view of the object. This seems conclusive, and when I 

 describe the apparatus I have no doubt the action of it will be 

 tested in England, and I would gladly entrust such trial to the 

 committee already according me their attention, and by all means 

 including Mr. Wenham. As a sort of explanation I am bound to 

 add that I am almost certain no English objective will be found to 

 go above 83° or 85° of balsam angle, which by the method I allude 

 to (which I doubt not will be accepted), those of mine of compound 

 fronts, will reach 90°, at all events in most cases, i. e. when the 

 air angle is 175° or upwards. 



More than this, the sort fairly denominated in your heading of 

 my article, " Peculiar Objectives," shall, in balsam angle, exceed 

 100°, and range above that according to the intention in their 

 construction. 



As yet I have done nothing about any further attestation here, 

 and when I do so it will be without knowledge of the opinions of 

 the gentlemen I shall call upon, or care either about their partisan- 

 ship of whatever theory. 



Eespectfully yours, 



KOBEBT B. TOLLES. 



P.S. — Since writing the above I have tested the angle of the 

 objective measured in London, and find the air angle at open point, 





