210 On Immersion Ohjedives. 



would at once see were justified, I never used assumptions, fair 

 or unfair, of Dr. Dawson's ignorance of the most familiar facts of 

 structural botany to bolster up my arguments. That did not need 

 to be assumed ; and nothing is, I hope, more certain to the reader 

 than that whatever were the familiar facta of strQctural_ botany 

 known to Dr. Dawson while I was yet a school-boy, the nature of 

 the wood cells of conifers, and the structure of the lower cryptogams 

 were not amongst them, and that he yet remains in ignorance of 

 these " familiar facts." 



IV. — On Immersion Ohjedives of greater Aperture than corresponds 

 to the Maximum possible for Dry Ohjedives. 



By Assistant-Surgeon J. J. Woodwaed, U. S. Army. 



I FEAE the readers of the ' Monthly Microscopical Journal ' are by 

 this time growing weary of the discussion of the angle of aperture 

 of immersion objectives, which has been going on so long in its 

 pages ; but I must beg their indulgence for this second appearance, 

 the question seeming too important to be left in its present un- 

 settled condition. 



My article published in the June number was in many respects 

 incomplete, in fact, only touched in the briefest manner (on page 272) 

 upon the optical considerations involved, for I sincerely hoped that 

 with the hints there given, Mr. Wenham would have arrived at the 

 same conclusions that I have done. I should have much preferred 

 that this had been the case, and that a correct explanation of im- 

 mersion objectives of greater aperture than corresponds to the 

 maximum possible for dry ones should have come from him ; for 

 I have a great admiration for Mr. Wenham, whose large services in 

 improvement of the microscope I fully recognize, and I regret to 

 find myself in antagonism to him, I need hardly say that I have 

 no sympathy with the personal aspersions of which he justly com- 

 plains,* and am sorry that anyone in America should have thought 

 proper to make accusations which I freely say I think as unjust, as 

 they can appear to him. Nevertheless I am obliged to infer from 

 Mr. Wenham's reply in the July number, that I have not made my 

 views perfectly intelligible, and certainly as I understand his, they 

 appear to me to be erroneous in several particulars, so that I feel 

 called upon to explain myself further. 



When I wrote my former paper, I understood Mr. Wenham to 



hold not merely that the angle of the extreme rays of the pencil 



transmitted through a given objective at a given position of the 



screw collar would be the same whether the medium in front was 



* July mimljcr of this Jouruul, j). 10 and p. 40. 



