Remarks on the Aperture of Object glasses. 271 



over 100° when the combination was fully closed. Measured with 

 the water tank, the angle at the uncovered point was about 130°. 

 Now, in the first place, I must remark that the objective was 

 certainly an exceptional one, and apparently put together with a 

 view to this controversy. Instead of three combinations, I found 

 it to be constructed with four ; the posterior two resembled those of 

 other fifths of Mr. Tolles, and were together moved by the screw 

 collar, the anterior two remaining stationary ; of the anterior 

 combinations the front was very small, and about a ninth of an inch 

 in solar focus. (It magnified 108 diameters at twelve inches' 

 distance from micrometer to screen.) Immediately back of this 

 was a very much larger combination, concave anteriorly and convex 

 posteriorly. I inferred from the manner in which the brasswork 

 was put together (having no information from the maker on the 

 subject) that these two combinations had been substituted for the 

 front of a previously constructed objective. 



In the next place I must remark that, notwithstanding its 

 exceptional construction, this objective, when used as an immersion 

 glass, had certainly very considerable defining power for a ith. It 

 worked, it is true, even when fully closed, only through the thinnest 

 covers, but it resolved the Amphipleura pellucida and Frustulia 

 Saxonica, both mounted in balsam (Moller's type-plate), and on 

 my Nobert's nineteen-band plate clearly separated the lines of the 

 fifteenth band. Used dry it would not work through any cover, 

 but when fully open it resolved the twelfth band of a Nobert's 

 nineteen-band plate, remounted with the lines uppermost and not 

 covered. In this performance the front of the objective appeared 

 to be in actual contact with the object. I may add that the 

 combination when in use magnified at twelve inches' distance sixty 

 diameters at the uncovered point, and seventy-five diameters when 

 fully corrected for cover. 



As the results of the measurements of the angle of the objective 

 last described are quite in disaccord with the sweeping opinion 

 expressed by my esteemed friend Mr. Wenham, in his recent 

 controversy with Mr. Tolles, I have thought it right to imitate his 

 prudent example,* and secure the testimony of competent witnesses 

 as to the accuracy of my results. I therefore repeated the measure- 

 ment of the balsam angle of this objective before Professor Simon 

 Newcomb, of the United States' Naval Observatory, and Mr. Eenel 

 Keith, of Georgetown, formerly also a professor in the same 

 institution. Both these gentlemen are professional mathematicians, 

 and both are well acquainted with optics as a science. They have 

 not only verified my measurement of the balsam angle of this 

 particular objective, but they agree with me that in the heat of the 

 discussion Mr. Wenham has gone rather too far in concluding that 

 * 'Monthly Microscopical Journal,' January, 1K~3, p. 29. 



