122 Further Remarks on Immersion Apertures. 



In concluding his article Mr. Wenham devotes a paragraph to 

 the defence of his mode of measuring the angle of Mr. ToUes' yV^h, 

 which requires a few words. It now appears that this immersion 

 objective was understood by Mr. Wenham to be a drj/ one. No 

 wonder he found that a dry objective of his own construction, made 

 twenty-two years ago, " proved far superior to Mr. ToUes', made 

 three years ago, on every object on which it was tested." * Nor 

 can I pass by the question of the position of the screw-collar used 

 in that trial, without calling attention to a fact which appears to 

 have been overlooked. Mr. Wenham says : "I should have been 

 quite content to try it, if the adjusting coUar had been pinned fast 

 by the senders in any position," &c., &c. Now, in Mr. Tolles' 

 original letter to Dr. Henry Lawson about this objective, pubhshed 

 in England four months before the trial,! he indicated the proper 

 position of the screw-collar to obtain the maximum angle of the 

 objective in what ought to have been as positive a way as if the 

 collar had been pinned fast. His words are — "Adjusted at 10^ 

 closed from the open point (which is the intermediate point of the 

 whole adjustment), I make the angle in balsam," &c. 



Now, as I have said in my last paper, I utterly repudiate all 

 accusations of bad faith against either Mr. Wenham or the gentle- 

 men who were associated with him in this unfortunate procedure, 

 but they certainly did not act as if they appreciated the importance 

 of the position of the screw-collar, and, in consequence, their results 

 have no significance as a check upon those of Mr. Tolles, and, 

 indeed, no scientific significance whatever, as they did not state 

 at what position of the screw-coUar they measured, but simply 

 measured at some undetermined position, selected, as it now 

 appears, by trying to find at which an immersion objective would 

 work best dry on a Podura scale. 



Finally, I must allude, briefly, to the letter by the Kev. S. Leshe 

 Brakey, in the December number, on the subject of my last paper. 

 This gentleman, who is more than usually acrimonious in his 

 language, finds no better way of defending his position than by 

 misrepresenting both his own remarks X and my reply. § I do not 

 think it necessary to do more than to request the candid reader to 

 compare the version of both, in his present efiusion, with the 

 original articles. 



War Department, Surgeon-General's Office, 

 Washington, D.C, December 30, 1873. 



* This Journal, January, 1873, p. 32. 

 t Ibid., September, 1872, p. Ii8. 

 t Ibid., August, 1873, p. 99. 

 § Ibid., November, 1873, p. 216. 



