128 Final Remarks on Immersion Apertures. 



upon to say a word or two with regard to certain matters which 

 have been pubhshed in this Journal during the last few months. 



And first, as to the following passage in Mr. Wenham's letter 

 in the April number (p. 171). " The immersion focus is therefore 

 the outer one, and the dry focus the nearest to the lens : now Col. 

 Woodward has drawn the reverse of this, and in his diagram made 

 the angle for immersion rays the inner one, or closest to the lens." 

 To this I reply, that the focal point F * of the balsam angle 82°, 

 corresponds necessarily to a dry angle only a differential less than 

 180°, in which the radiant must be at a point infinitely near W on 

 the front of the objective, and that therefore in my drawing the 

 " immersion focus is," in point of fact, " the outer one, and the dry 

 focus the nearest to the lens," as Mr. Wenham says it ought to be, 

 notwithstanding the circumstance that the foci of the correspond- 

 ing balsam angles occupy precisely the opposite relations. 



The next point to which I desire to refer briefly, is Mr, Wen- 

 ham's new method of measuring apertures t by placing a vertical 

 slit in the focus of the objective. This method might perhaps be 

 used without giving rise to material inaccuracy when the objective 

 is adjusted for uncovered objects ; but when it is closed to the point 

 of maximum aperture, that is in the very position about which 

 alone there is any dispute, its spherical aberration is of course no 

 longer corrected for uncovered objects, and if the attempt be made 

 to focus upon them, as for instance upon the glass between the jaws 

 of Mr. Wenham's slit, it is quite possible to make such errors in 

 endeavouring to approximate the correct focal distance as to destroy 

 the accuracy of the result, and lead to the unintentional cutting off 

 of more or less of the actual angle of aperture. 



Lastly, I feel compelled to refer to the paper by Mr. Brakey in 

 the May number of this Journal (p. 221). This writer, whose 

 peculiar style of wit owes whatever poignancy it may possess to the 

 ready unscrupulousness with which he misrepresents the views of 

 those whom he attacks for the purpose of trying to make them 

 appear ridiculous, devotes four pages to a comic presentation of my 

 opinions, and of the part I have taken in this discussion, which is 

 characteristically inaccurate. If I could suppose that he had mis- 

 understood my former articles, I should feel called upon to explain, 

 but as it is evident that the misrepresentations are intentional, I 

 shall accord to the article no further notice than this brief para- 

 graph. 



* See diagram facing p. 213, this Jounial, November, 1873. 

 t This Journal, March, 1874, p. 112, and May, 1874, p. 198. 



