MEMORANDA. 161 



January, 1855, the very time in which Mr. Wenham's retrac- 

 tion of his remarks appeared in the ' Quarterly Journal 

 of Microscopical Science.' 



If Mr. VVenham finds anything objectionable in the form of 

 my reply, he should bear in mind that the discussion is not 

 one of my seekinoc, and that I put the best possible construc- 

 tion upon his remaiks which called in question the coi'rectness 

 of my assertions. I am utterly averse to anything like scien- 

 tific controversy, and would make no further remarks in this 

 connection, if Mr. Wenham had not so entirely mistaken my 

 statement, as to represent me as having published sheer 

 nonsense. 



The statement on which Mr. Wenham animadverts is as 

 follows : " The error in Mr. Wenham's arguments Avill be suffi- 

 ciently obvious to any one who will trace the course of a 

 divergent beam out of the balsam, instead of into it; and it 

 will then be seen that large angles of aperture are as useful 

 for balsam-mounted specimens as for others." This state- 

 ment, as it stands, I still hold to ; but I must protest against 

 its being considered as " tantamount" to any such absurdity 

 as that into which Mr, Wenham has translated it, which 

 is indeed " contrary to reason." I mean, however, to assert 

 what Mr. Wenham so emphatically denies, viz. : that it does 

 make a difference, whether rays are traced into a refractive 

 medium or out of it. I cannot admit that these two cases 

 " come to precisely the same thing." 



Mr. Wenham surely does not need to be told, that if " the 

 trigonometry of optics establishes anything, it proves that the 

 same medium which bends an incident ray toicards the per- 

 pendicular when it enters, will bend it from the perpendicular 

 when it emerges. Hence a beam of divergent rays, from a 

 point within a medium, is rendered still more divergent when 

 it emerges, and in fact is spread out, so that the extreme rays 

 which emerge are in the plane of emergence, or make an 

 angle of 180° with each other. 



Mr. Wenham seems to confine his attention to the fact, 

 that a large portion of the rays from a balsam-mounted object 

 are lost by internal reflection. This, of course, I never meant 

 to deny ; and, in fact, it is one obvious reason why balsam- 

 mounted test objects are, as I long ago stated, far more difficult 

 to resolve than when mounted dry. The loss of a portion of 

 rays in this manner, however, has nothing whatever to do with 

 the present question, which is simply whethei', of the rays that 

 do emerge ('and which make every angle with each other, from 

 0° to 180°) more will be collected by a lens of large or small 

 aperture. Certainly Mr. Wenham cannot deny that the larger 



VOL. IV. M 



