104 UPON INSTRUMENTS CALLED PERISCOPIC. 



Observation same diameter and focus as the meniscus, has less spherical sur- 

 the meniscus is *" ace ' and consec l uent ly less longitudinal and lateral aberration of 

 inferior in its the two. Let us now advert to the transformation of the con- 

 effect to the vex j ens to become a meniscus, with the same focus : by con- 

 double convex . 7 J . 



lens. sidenng their figures in the diagrams, the reader will perceive, 



that as much as the upper surface of the convex has been incur- 

 vated for a meniscus, so much the more has the convexity of 

 the under side been augmented, to retain the original focus. 

 The oblique pencils of rays first entering the meniscus, or any 

 part of its surface, are from the immutable law of refraction 

 refracted from the axis of the lens, contrary wise to the first di« 

 - rection on the convex, and afterwards in their passage into air, 

 by the increased inferior convexity, refracted back towards the 

 axis proportionally more than by the under side of the double 

 convex to be converged to the same focal distance ; and all 

 pencils of rays that impinge on the surface in an oblique direc- 

 tion to its axis, must be united the same as by the convex lens, 

 at a focus somewhat shorter than the principal focus from 

 direct rays. The meniscus lens, in refractive property, differ* 

 not from the double convex one. The above explanation is 

 agreeable to all writers on optics, and to correct experiment. In 

 this meniscus, it is not " if the incidence,*' &c. but the inci- 

 dence always is so oblique on the second surface, as to increase 

 the convergence ; and no kind of opening E whatever will 

 change nature's laws of refraction, so as to elongate the focus, or 

 to produce two different focuses in one lens j and his previous 

 explanation of " occasioning all pencils to pass, as nearly as may 

 be, at right angles to the surfaces of the lens," page 27, is an 

 irrelevancy in optics, and is the error of reasoning that I imputed 

 formerly to Dr. W. on his spectacle glass. It is the angle that 

 the rays make with the axis of the lens, of whatever shape, that 

 refraction is estimated from, as the science teaches us ; not 

 from the geometrical positions of pencils and surfaces. From 

 the greater aberration that the meniscus possesses, the images 

 formed by it will be less distinct, have less light, and be more 

 distorted than by the double convex lens. It is from the extended 

 lateral distortion, and bringing the meniscus nearer to the 

 plane than its exact focus, that I can assign a cause how Dr. W # 

 could have fallen into the error $ had he placed the concave side 

 downwards, it would have been a better position, the images 



woulcj 



