UPON INSTRUMENTS CALLED PERISCOPIC. 105 



would have been more defined and enlightened ; it was so 

 applied in his spectacles, the convex side being next to the 

 object : but in neither case will the images be so perfect and 

 vivid, as by the double convex lens. The meniscus in a The meniscc* 

 Camera is not a new application j several, some years back, ^ *,^™ er R e . 

 were made for the purpose, but not preferred. I can refer to ferencc to ex* 

 the machine now existing with one. I have caused two lenses penment. 

 to be ground, one a double convex, the other a meniscus, as 

 J3r. W. directs, of the same diameter, nearly four inches, and 

 focus twenty-two inches ; which experimentally verify the cor- 

 rectness of my observations, and which any intelligent person 

 may inspect, by application at our manufactory, 30, Hol- 

 born. 



The following quotations may to some of your readers better Quotations re* 

 , . , r i specting lenses, 



corroborate the truth of my remarks. 



U If the side were concave (of a piano) so that the lens be- 

 came a meniscus, there is no proportion of the radii, or position 

 of the lens, with regard to the radiant, but what will give the 

 aberration greater than the piano convex in its best position j 

 and, since this was first observed by opticians, the meniscus be- 

 gan to lose ground in the construction of optical instruments, 

 and is now quite rejected." Martin's Elements of Optics, 17$9, 

 page 29. 



An oblique pencil of rays has its focus a little nearer the lens 

 (double convex) than a direct pencil. Cor. fig. 2. 



This prop, holds good in a concave lens, and also in a 

 meniscus, as well as in a convex one. Emerson's Optics, paga 

 J24, prop. 24. 



" When parallel rays fall upon the plane side of a plano- 

 convex glass, the aberration of the extreme ray, which is & 

 of the thickness, is less than the like aberration caused by any 

 meniscus glass whose concave side is exposed to the incident 

 ray. 



" When the said glasses have their convexities turned to 

 the incident rays, the aberration of the extreme ray in the 

 plano-convex, which is now but ^ of its thickness, is less than 

 the like aberration of any meniscus in this position." 



The best of all double concave glasses has the semi-diameters 

 of its first and second concavities as 1 to 6 j and consequently, 

 this is the best figure of a glass to help short-sighted persons, as 



th*. 



