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XXIV. On Vision. By Ez. Walker, Esq., of Lynn, 



Norfolk. 



To Mr. Tillock. 



- -SIR, 



iVxy paper on Vision, printed in the 29tb volume of the 

 Philosophical Magazine, has been reviewed in one or two 

 of the periodical publications. 



" We," says one of these writers, *' cannot help regard- 

 ing it as, in some measure, derogatory to the character of a 

 respectable journal, and a» the same time discreditable to 

 the literary reputatioti of the country, that papers should be 

 brought forward, without censure, and without comment, 

 which betray the deficiency of their authors in the first ele- . 

 ments of science." 



I believe it is i o\v preitv well understood, that nothing is 

 so " discreditable to the littrary reputation of the country," 

 at this time, as the concealed, ignorant critic, who passes 

 an unjust censure upon the works or others. 



" It is true," savs this reviewer, " that when the surfaces 

 of a lens are perfectly spherical, its mean focal length Is 

 altered in a very slight degree by a change in its aperture; 

 but this change is in all practical cases absolutely insensible, 

 and, unfortunately for M'. Walker's opinion, is of a nature 

 precisely opposite to that which he takes for granted." 



But, whatever may be the opinion of this reviewer, it 

 will be generally believed, that the bare assertion of an ano- 

 nymous writer does not alter the truth of any proposition. 



The truth of what I have advanced in my paper, respcct- 

 ino- this property of the convex lens, may be clearly under- 

 stood from the following 



Experiment. — I took the same instrument mentioned i» 

 mv former paper, and directed it (o the moon, and drew out 

 the inner tube until her ima2;e appeared distinct upon the 

 unpolished glass. Tiien, after having contracted the aper- 

 ture of the lens from two inches to J- of an inch, the image 

 of a candle, which stood at the distance of 12 feet from the 

 instrument, was distinctly painted, in an inverted position, 

 upon the alass in the lunar focus of the lens. 



The 



