CORRESPONDENCE. 133 



the difference of refractive power changes, aud the limit, which also 

 depends upon the difference of refractive power, changes with that 

 variable. The only case in which the limit can remain the same is 

 when the effect of change in the two variables is in opposite directions. 

 In the case of an ordinary immersion objective the effect of the change 

 in both variables is to increase the angle of total reflexion, whatever 

 fluid is substituted for air between the surfaces. 



The result is that 82° is no longer the theoretical limit of aperture 

 under these circumstances, but a limit far greater, depending, of course, 

 in some degree, upon the fluid used. For balsam it is 180° theoreti- 

 cally ; what the opticians can do with it practically remains to be 

 seen. 



It seems to me that it is only from want of attention that Mr. 

 Wenham has failed to take this very clear and easy abstract view of 

 the matter. 



Eespectfully, &c., &c. 



Eenel Keith. 



Mr. Stodder's Last Letter. 



To the Editor of the ' Monthly Microscopical Journal.'' 



Padnal Hall, Essex, January 31, 1874. 

 Sir, — Mr. Stodder's letter merits no reply further than to correct 

 a misprint. For the thickness of cover referred to, instead of ^^th 

 read yV^^^ of an inch. My authority is taken from page 97 of this 

 Journal for Aug., 1873. 



As I need not trouble my readers with remarks useless in the dis- 

 cussion of scientific truth, I leave Mr. Stodder in his appropriate garb 

 of personalities. My argument has been preferably transferred to 

 Colonel Woodward, from whom no such insinuations can come.* 



Youi's truly, 



F. H. Wenham. 



* My explanations concerning this object-glass trial have brought invective 

 from Mr. Stodder. In his eagerness to attribute dishonesty of intention to me, 

 he overlooks the fact that the extraneous question of performance may be set at 

 rest. No offer comes from him, but if he so desires, and will again send the 

 object-glass by hands that he can trust, I shall be pleased to have it tested as 

 an immersion, with any thickness of cover that he may choose, against my old 

 dry Jth as before, in the presence of a number of witnesses. It has not been 

 considered proper amongst microscopists to hold public meetings, in order to pass 

 judgment on the respective merits of object-glasses. In this I quite agree, 

 but these insinuations induce me to make this offer, which can bring no opposition 

 as the curves of this |th are now obsolete. 



I am not a trader in object-glasses, but a teacher in their construction, and it 

 is, I hope, with excusable pride that I see, as a consequence of my voluntary con- 

 tributions (the series of which have recently been republished), amateurs not 

 only here, but in America also, taking up this elegant manipulation with success. 

 I am under no condition that binds me to reserve, and shall have further par- 

 ticulars to disclose. 



