UPON INSTRUMENTS CALLED PERISCOPIC. 101 



pie, and affording an improvement in the figure of a spectacle 

 glass, was no other than the old rejected Meniscus Lens j con- 

 tained no principle of refraction, different from the plano-con- 

 vex, and double convex lenses j but, as it caused a greater 

 aberration of the rays of light than those two lenses, was a 

 worse form of lens for spectacles, or any other instrument, than 

 the double convex lens generally used by practical opticians : It 

 must, therefore, surprise others, besides myself, that Dr. W. 

 should be induced again to propose the Meniscus, as an im- 

 provement in the Camera Obscura, by substituting it for the 

 double convex lens, his account of which you copied last month 

 into your Journal, from the Philosophical Transactions for 

 1812. 



The desire I have to maintain an optical truth, and the duty Motive of tht 

 -r /- . i . , ,. . • present me- 



I owe to our professional interest, obliges me again to point out m oir. 



to your readers, what I judge to be the error of his reasoning, 



and the fallacy of the inference. 



In his description of the effect of the double convex lens in Remedy stated 



the common camera, page 27, he states the known effect of the j a s t0 n ' a3a pJi{. 



images distant from the middle, or direct focus of the lens, being cable to the 



somewhat indistinct, on account of the plane of representation common . Ca * 



r mera ; viz. to 



becoming, in distance, greater than the principal focus of the place thescreen 

 lens j and the oblique pencils of rays being refracted to a focus, y/l } hn } th * 

 rather shorter than the principal one. " On this account," he cus. 

 adds, u it is in general best to place the lens at a distance some- 

 what less than that which would give most distinctness to the 

 central images, because in that case a certain moderate exten- 

 sion is given to the field of view, from an adjustment better 

 adapted to lateral objects, without materially impairing the 

 brightness of those in the centre." The aberrations of the lens 

 add also to the indistinctness. 



The collateral indistinctness in our portable chest Cameras, is OkJ" 11 ""' an £ 

 , . . ■ ► description of 



but trivial and unimportant ; and, in my opinion, the remedy, the remedy in 



as above proposed, will be found by the artist to be worse than use 5 viz - to 

 *u j c . a j. .• , . . , , . ,, , makethescreen 



the detect, as the distinct and vivid central images will be C oncav». * 



vitiated, and the extreme images but little improved. The most 



perfect remedy is that which has been used by opticians in large 



cameras, for more than 50 years past, of placing a bottom 



board, or whitened table, with a concave surface, proportionated 



to the focal distance of the lens j which, corresponding very 



nearly 



