ic6 



VtOK INSTRUMENTS CALLED PERISCOTIC. 



the double convex, one of the like figure is the best for specta- 

 cles." Smith's Optics, Art 66 1, 662. 664. 



*' For since a meniscus, unless the surfaces of it are parallel 

 to one another, has the same effect cither that a convex lens, 

 or a concave one would have, all the cases of diverging or con- 

 verging rays that are refracted by it, will be the same with those 

 already explained in the instances of convex or concave lenses." 

 Rutherford's Philos. vol. 1, page 286. 



" A plano-convex glass, with its < ' . a > side towards 



the incident parallel rays, has less aberration than any meniscus 



with its < > side exposed to parallel rays. Whence 



(concave > ? r j 



it necessarily follows, that that meniscus is best, which ap- 

 proaches ncaiest in shape to a plano-convex lens." Harris (of 

 the Mint) Optics, 1 77(5, p. 67. 



So sensible have some optical glass grinders been of the im- 

 practicability and insufficiency of the meniscus glasses of short 

 foci for spectacles, that I have in my possession some plano- 

 convex and plano-concave glasses actually fitted in the frames, 

 and sold for the neiv periscopic glasses. 

 Observations The sort of French angle of reduction that Dr W. has given, 

 ©n the peris- to obtain geometrically but nearly the radii of meniscus for a 

 given focus, will be useless to the workman, as he aheady 

 knows, by a very short arithmetical operation, how to obtain 

 exactly such radii in half a minute's time, or a tenth part of the 

 time necessary to construct that problem by Gunter's sliding 

 rule, the time would be still shorter. 



The combination of using two glasses in ordinary simple 

 microscopes, or hand magnifiers, to diminish the errors arising 

 from the spherical figure of one glass, was known to Sir Isaac 

 Newton, and successive opticians. That late excellent practical 

 optician, Mr. Ramsden, by the combination in the best position 

 of two piano glasses, with their convex sides to each other, ap- 

 plied eye-pieces to his instruments with great advantage, to read 

 off divisions of his circles, and magnify the wires ot his teles- 

 copes, with clear definition at the circumference of the field of 

 view, the diameters of the glasses being no smaller than the 

 aperture of the tube. The same principle has since been advan- 

 tageously applied to large object lenses for the lucernal micro- 

 scope, 



«or.ic micro- 

 scope 



