CORRESPONDENCE. 115 



extravagant apertures.* This point might still give rise to long 

 disputes, but confined to the subject of Mr. Tolles' last communication 

 on " the optical quality of his (Mr. Tolles') ^th objective," there remains 

 the singular fact, that the diameter of the front lens compared with 

 the focal length proves the aperture endorsed on the mount to be 

 simply impossible. There can be no reasoning against this but to 

 assert that my dimensions of diameter of front lens, position, and focal 

 distance are untrue. Then I have no doubt that the challenge will be 

 accepted, the measurements repeated, and comparisons made by other 

 hands than mine. 



I do not wish to shirk the discussion of the aperture if any further 

 improvement can result from it, but if I am to be at liberty to select 

 my antagonist, it will not be one that affirms that 180° is possible, or 

 that even 179° is practicable. 



In justice to Mr. Tolles I will say that throughout all the argu- 

 ment he has comported himself with exemplary good humour, not even 

 resenting the chaif that he has endured during the discussion. A con- 

 trast to the conduct of his acting agent, who, without a ray of science 

 to enlighten the subject, or to justify his authority, has done nothing 

 but throw dirt at all opponents, of si:ch an odour, as to cause them to 

 hurry j)ast on his windward side rather than stop to argiie with him. 



I am, Sir, your obedient servant, 



F. H. Wenham. 



Eeduction of Aperture in Object-glasses of Telescopes. 



To the Editor of the ^Jllonthly 3Iicrvscoj)ical Journal.^ 



Boston, U.S.A., June 19, 1874. 

 Sib, — While, substantially, declaring it unlikely that any intelli- 

 gent reader of the Monthhj would confound contraction of field in an 

 eye-piece with reduction of aperture in the object-glass (and with 

 which I fully agree, but must suggest the irrelevance of the allu- 

 sion to beneficial reduction of aperture in object-glasses of ielescopes'\ 

 as of some sort of parity to this case of the microscopic objective), 

 Dr. Pigott in concluding his note (your Journal for June, p. 268) 

 says, " that in a limited field of view the definition ... is superb ; 

 beyond the central area the definition is very indistinct. This seems 

 to indicate that the very oblique pencils are not as free from aberra- 

 tion as the central." While thanking Dr. Pigott heartily for having 

 discovered that "very oblique pencils" were concerned at all in form- 

 ing the image with that objective (I) I am bound to suggest that 

 when we contract the field-bar of the eye-piece, the " central area " 

 continues to have all the rays, " very oblique pencils " included, that 

 reached that central area before contraction. This does not " seem to 



indicate," &c. xr xi- n 



' Yours respectiully, 



E. B. Tolles. 



* In my diagi'am, p. 114 of Journal (March, 1874), I had " drawn" ray d cor- 

 rectly, but the final deviation withorrt the normal appeared so small on the 

 reduced scale that the wood engraver failed to notice it, and continued the line 

 straight. 



t " This everybody knows well enough ! " 



