COliRESPONDENCE. 271 



and wliicli are suijplied to the most minute objects \vitli perfect 

 success. 



It would be unbecoming in me to question any of tlie optical 

 opinions of Sir David Brewster, but I think I may venture to break 

 a spear with Dr. Pigott, even under the £egis of so eminent an 

 authority. 



It will be admitted, I presume, that the arguments of Sir David 

 Brewster, so far as they relate to ^jhotography (of which alone I am 

 treating), rest entirely uiion the analogy between the camera obscura 

 and the human eye ; and I agree that there could be no better founda- 

 tion upon which to proceed ; but in reasoning out the parallel between 

 the two, Sir David, by a most uuaccouutable oversight, has entirely 

 overlooked the principal factor in the question, namely, the length of 

 the ocular focus. 



If the oculaT focus be assumed as • 8 of an inch, represented by a ; 

 the aperture of the pupil O* 2 inch, represented by h ; the focus of the 

 camera obscura 12 inches, re2)resented by c ; then it is evident that, 

 in the human eye, the aperture of the pupil is one-fourth of the ocular 

 focus ; and the proportionate aperture in the camera lens would be 



12 . 



— = 3 inches, not ^jj of an inch, as assumed by mistake. 



Or, substituting the real or any other measurements, and adopting 

 the symbolic but accurate and comprehensive language of Dr. Pigott, 



the proportionate aperture would always be — • 



I do not suggest that a trifling oversight of this nature will in the 

 slightest degree affect the scientific character of Sir D. Brewster ; but 

 it vitiates the whole reasoning of which it is the foundation, and 

 throws some light upon the justice and propriety of certain " unsparing 

 criticisms." 



Finally, I feel entitled to call upon Dr. Pigott, either to point out 

 the error in my calculation, or to accept it as true. In either case, I 

 apologize beforehand for any expression I may have used which may 

 have given him pain. 



Your very obedient, 



G. S. CUNDELL. 



P.S. If not encroaching too much upon your space, now that I 

 have cleared my stomach of controversy, 1 should like to say a few 

 words in extenuation of the faults of certain unskiKul practitioners in 

 photographic portraiture, who have brought so much obloquy upon 

 their whole fraternity : I must, however, inform Dr. Pigott that I have 

 nothing to do with that beautiful and now very important art. 



Everything may be overdone, and certainly everyone must occa- 

 sionally have seen in photographic portraits an undue prominence 

 given to stereoscopic eifcct. I am, however, by no means in favour of 

 abolishing it altogether, by the use of lenses of j-jy of an inch aperture. 



Do we not, to our great advantage, see everything stereoscoi^ically ? 

 Painters, I believe, paint all that they can see, and, as they use both 



