CORRESPONDENCE. 171 



just in a question of simple rigid optical law, where there ought to 

 be no indecision. 



It would have been in favour of Col. Woodward's demonstrations 

 if I had ignored the fact that the screw-collar adjustment alters the 

 aperture of the object-glass and focal distance. The closing of the 

 lenses lengthens this from the anterior surface of front lens, and for an 

 immersion object or one under a* very thick cover the lenses are 

 always brought closer. 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 

 ■'mmersion rays the inner one, or closest to the lens. This is the first 

 view that may be taken of the demonstration. Much knowledge has 

 been recently acquired of the internal construction of microscope 

 object-glasses ; and I should have been glad if Col. Woodward had 

 given us an illustrative figure, having some relationship to a reality 

 with the rays carried to their final destination. The passage of all his 

 rays should have careful consideration, and if I saw no error I could 

 not state that there is one, and trust that I have the candour to admit 

 accuracy. My " mere assumption " had no reference to the construc- 

 tion and position of mythic back lenses, for I challenge the assertion 

 that rays from the two foci that he has shown, can be got through the 

 same lenses of any combination and both form a true focus at the back. 

 Considering the optical merits of Col. Woodward's diagram, I ask 

 whether he himself is not liable to the imputation he aj)plies to me, 

 for putting forth such evidence with the absence of all-important 

 back lenses, relative to which he finds fault with me for not knowing 

 the construction and position of? The time must at length arrive 

 when the substantial facts brought to light by this question must be 

 weighed and recorded against mere words, and then the balance may 

 be in my favour. 



With reference to other correspondents, this long discussion 

 has on some occasions so inconveniently occupied my time, that in 

 future I must confine myself to the main points of fact and demonstra- 

 tion, and not answer frivolous objections or personalities, that I little 

 care for. 



I remain yours truly, 



F. H. Wenhaji. 



Eev. S. L. Brakey or Mr. Wenham ? 



To the Editor of tJie 'Monthly MicroRCopiod Journal.'' 



Sir, — In the current number of your Journal, Dr. Woodward 



briefly alludes to the " more than usually acrimonious language " of 

 your correspondent, the Kev. S. L. Brakey. Now, sir, this latter 

 gentleman has exhibited a great fondness for dealing in criticism ; 

 and as he distinctly says that on the aperture question he knows that 

 " substantially " his " observations must coincide with " Mr. Wenham's 

 VOL. XT. 



