Letter from Ezeklel TValker, Esq. 335 



lliat tlie author ought fo have known, before he began to 

 write on optics, that the demonstration of the theorem, 

 which is the foundation of u)y Theory of Vision, is to be 

 seen in ahnost every book of optics that has been written in 

 thcEnghsh language since the days of Newton; and tliat I 

 have not made a greater " parade of algebraical substitu- 

 tions" than those writers have done. He ought also to have 

 known, that it is the cornn)on practice of niaihematicians to 

 use a well known theorem without its demonstration. 



Secondly, if what is called a demonstration contains any 

 one step that '^ is vague and loose," it is no demonstration 

 at all. Had this reviewer of philosophical papers found any 

 step in my demonstration that is not strictly .ind rigidly true, 

 he ought to have pointed it out, and advanced some belter 

 arguments against it than Ids oivn bare word. 



Thirdly, " The aberration in any lens by the different re- 

 frangibility will be ncailv the same as in a plano-convex of 

 ihe same focus. 



*' For in a double convex lens of equal radii, the refraction 

 at each surface will be but half so much as at the convex 

 surface of the planoconvex, which has double the curva- 

 ture*." 



Whence it is evident that I did not shift my hypothesis 

 before I drew the conclusion. 



Lastly, As I never saw Berkeley's New Theory of Vision, 

 I could not draw any facts from it ; my theory is founded 

 on the discoveries of a greater philosopher than even bishop 

 Berkeley. 



I am, sir. 



Your most humble servant, 

 Lynn, Ez. Walker. 



Sept. 17, 1807. 



♦ Emerson's Optics, p. ITS. 



Z2 LVIII. lit. 



