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IV. — Remarks on the Aperture of Object-glasses. By Assistant- 

 Surgeon J. J. Woodward, U. S. Army. With a Note by 

 F. H. Wenham, V.P.E.M.S. 



Plate XIX. (Lower portion). 



February 17, 1873, I received a note from Mr. A. B. Tolles of 

 Boston, asking me if I would measure the balsam angle of a T Vth 

 objective for him. Having agreed to do so, the objective came to 

 hand before the close of the month. My intention was to measure 

 the angle by the modification of Lister's method proposed by Mr. 

 Wenham,* and afterwards used before a committee of scientific 

 gentlemen in measuring the T Vth rashly sent by Mr. Tolles to 

 London for that purpose.! Mr. Tolles, therefore, at my request, 

 supplied a sector and tanks. 



Having had some previous experience with the ordinary method 

 of measuring angles of aperture with the sector, I was well aware 

 of the erroneous result likely to be obtained by its use in the 

 case of high angles, but supposed that for the reduced angles to be 

 measured, when the nose of the objective was immersed in water or 

 balsam, it would prove at least as nearly accurate as for similar 

 angles measured in air. I soon found, however, that this was not 

 the case, if the screw collar was fully closed. 



I first measured the T Vth sent by Mr. Tolles with the screw 

 collar adjusted to the open point, that is, for uncovered objects. 

 The sector, used precisely as described by Mr. Wenham, gave the 

 angle in air at 160°. When the nose of the objective was im- 

 mersed in a tank of water, the angle was reduced to 9 3 d , and in 

 fluid balsam to 76°, as nearly as could be read by the sector. 

 When, however, the screw collar was adjusted for the thickest cover 

 through which it could work, that is, when the combination was 

 closed as far as possible, I failed to get definite results either in air, 

 water, or balsam. At no angle was the field of view bisected 

 fairly, bright on one side and dark on the other ; but the light 

 gradually faded away in such a manner that no sharp limit could 

 be fixed. 



I did not feel at liberty to escape this difficulty as Mr. Wenham 

 did, in measuring the Tolles's T J oth sent to London, by setting the 

 screw collar at some more open point (" the best adjustment of a 

 Podura scale," for instance), for I had found by trial that when the 

 lens sent to me was closed as far as its screw collar would go, it 

 would still define very well, provided it was used on an object covered 

 by a correspondingly thick covering glass. Worked at this adjust- 

 ment the lens, in fact, would show the beads of Pleurosigma angu- 



* ' Moutlily Microscopical Journal,' August, 1872, p. 84. 

 t Ibid., January, 1873, p. 29. 



