foO Critical Olservations on Dr. IVullasion's 



like that of the mtiiiscus ; nor has he told the reader what 

 improvement would be produced, if he placed a similar cir- 

 cular opening, or limited aperture, also over ihe convex 

 lens. I nui>t. therefore, inform the reader; and he may 

 himself prove it to be correct. The diameter or aperture 

 of four inches is too great for a lens of tweniy-lwo inches 

 focus, either double convex or meniscus lens, placed in a 

 camera obscura ; as it transmit? too much light, and pro- 

 duces too much aberration for the most distinct representa- 

 tion of the images wiihin the camera. Dr. Wollaston 

 therefore, no doubt, was obliged to correct this palpable 

 defect, by a curtailment of the area of his lens no less than 

 //iree-/b2<r//!5 of the whole, and the lens would have been 

 more like one applied by a skilful optician, if he had at 

 first inserted a lens of about two inches diameter. The 

 limited aperture therefore, it is evident, advantageously ex- 

 cludes rays, but has nothing to do with the determination 

 of their direction. Upon a fair comparison, the reader will 

 not only doubt of the superiority of Dr. WoUaston's ca- 

 mera, but he convinced of its absolute inferiority; for the 

 double convex lens, under the same diameter and focus as 

 the meniscus, has less spherical surface, and consequently 

 less longitudinal and lateral aberration of the two. Let us 

 ' now advert to the transformation of the double convex lens 

 to become a meniscus with the same focus : by considering 

 their figures in his 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 ravs first entering the me- 

 niscus, or any part of its surface, are, from the immutable 

 laws of refraction, refracted from the axis of the lens, con- 

 trariwise, to the first direction of the convex, and, after- 

 wards 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 he 

 converged to the same focal distance ; and all pencils of 

 rays that impinge on the surface in an oblique direction 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, 

 differs not from the double convex one. The above ex- 

 planation is agreeable to all writers on optics, and to cor- 

 rect experiment. In this meniscus it is not " if the inci- 

 dence," &c. but, the incidence ahimjs is so oblique on the 

 ECLcnd surface as to increase the convergence; and no kind 



