( 199 ) 

 COEEESPONDENCE. 



Mr. Wenham's Criticism on Professor Keith's Diagram and 



Computation. 



To the Editor of the ' Montldy Alicyoscopicul Journal.' 



Alexandria, Virginia, January 15, 1876. 



Sir, — It seems to me desirable to replj to some of Mr. Wenham's 

 remarks * in relation to iny computation of tlie angular aperture of 

 the Museum ^^jjih, lest his errors should prevent some from seeing the 

 full force of the result. 



With regard to Mr. Crisp's Jlth, it was perfectly immaterial to me 

 which of the objectives, having the aperture ridiculed by Mr. Wenham, 

 was taken up. And I am now perfectly willing to take up Mr. 

 Crisp's, if anyone desires it, and have no doubt that it would also 

 illustrate the same statement, viz. "that the so-called theoretical limit" 

 to the amount of light that can pass out of glass into air, has nothing 

 whatever to do ivith the aperture of immersion lenses. 



My diagram, which Mr. Wenham dismisses in a sentence quoted 

 by Mr. Mayall, jun., represents correctly (as stated in the papers 

 accompanying it) the lenses of the Museum -j^th, an oiyective which 

 is not known at the Museum to have been surpassed by any other of 

 the same power. Mr. Wenham's guess, that it could not be focussed 

 upon a dry object, is directly contrary to Dr. Woodward's statement 

 in his accompanying paper,f viz. " it performs admirably as a dry 

 lens." The diagram further rejn-esents the path of a ray of light, 

 which is of course " drawn in accordance with the computed results.' 

 It however fails to satisfy Mr. Wenham for the very curious reason 

 that it " suits the proposition," whatever that may mean. The result of 

 the computation is sufficient evidence that the curves, distances, and 

 refractive indices were correctly given by the maker, otherwise, the 

 objective could not have been found free from sjiherical aberra- 

 tion. But in addition to this the lenses were unscrewed by Dr. 

 Woodward and myself and the maker's elements verified by measure- 

 ment, as far as it was possible to do so, before the computation was 

 undertaken. 



The fact that the diagram represents correctly the well-known 

 Museum x^h, the photographs taken by which have given such 

 general satisfaction, adds interest to the paper. But if elements had 

 been guessed out, free from spherical aberration, the force of the result 

 would have been the same. 



Mr. Wenham's attempt to fix the limit of mathematical computa- 

 tion is quite as amusing as his attempt to fix the limit of aperture. I 

 can assure him that it is perfectly possible to compute the spherical 

 aberration of any combination of lenses however complicated and with 



* ' M. M. J.,' Nov. 1874, p. 221 . t ' M. M. J.,' Sept. 1874, p. 127. 



