COERESPONDENCE. 267 



against their own, will, I think, agree with me that the last is superior. 

 I may mention that this has a "doublet front" consisting of two 

 single plano-convex lenses, as some discussion of the merits of such a 

 form both in theory and practice may be appropriate, and result in 

 some useful knowledge. 



I must remind my readers that the first definite information on the 

 subject of the modern object-glass was published by myself in the early 

 numbers of this Joui'nal, in what had before been rather a matter of 

 secresy. And on the information derived from my writings, I 

 scarcely expected that I was to be blown up with my own petard. 

 But so long as the fabric is begtm from the wrong end, it must 

 finally fall. We all know how the achi-omatic telescope has benefited 

 by the labours of Herschel, Barlow, and Airy ; their foimulfe were 

 worked from a true basis of parallel rays, and the aberrations con- 

 sidered relative to their path to a final focal point, thus beginning 

 from a definite and known position. So it should be with the micro- 

 scope object-glass, for in order to treat it diagramatically and mathe- 

 matically, the course of the rays must be commenced from the long 

 conjugate focus at the back, near the eye-piece, which can be taken 

 as a correct and absolute position, and in a theoretically accurate 

 combination the anterior focus must then follow with certainty. To 

 attempt to arrange a construction from an assumed focal point in 

 front, in hopes of defining the aberrations, is about as senseless as 

 trying to build up a pyramid from its apex. It is no wonder, then, 

 that those attempting this get no further than the front lens, and there 

 abandon the argument. So long as this coui'se is adopted, the debate, 

 thus starting from a baseless foundation, can never arrive at a con- 

 clusion, and it is therefore useless to continue it. 



I anticipate that this peremptory dismissal may bring its comment 

 in the late style ; I will, however, add that I shall respect a diagram 

 illustrative of the immersion principle, in which the back combinations 

 serve the twofold purpose, and where in both cases the rays must 

 follow the same coui'se and direction up to the front lens — for the 

 immersion principle lays beyond this. I will take into consideration 

 any correctly-drawn demonstration that will show a result. 



I am yours very truly, 



F. H. Wenham. 



KusTicus, JUN., ON Mr. Brakey's Eeply. 



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

 Sir, — Believing the Eev. Mr. Brakey to be willing — nay, anxious 

 to try his dialectic on any question involving an " Optical Curiosity of 

 Literature," I ventured to request his consideration of the two passages 

 which I placed in juxtaposition : the one written by himself, the other 

 by Mr. Wenham. Instead of reconciling the statements — his own 

 and Mr. Wenham's — which he had previously said he knew " must 

 coincide," he lays down this paradoxical theorem : " In the two pencils 

 \i. e. with immersion— and with dry lens] light is lost unequally, one 



VOL. XI. X 



