r& *ERISC0PJC camera and microscop*. 



aperture of the lens is four inches, its focus about twenty-two. 

 There is also a circular opening, two inches in diameter, placed 

 at about one eighth of the focal length of the lens from its 

 concave side, as the means of determining the quantity and 

 direction of rays that are to be transmitted, 

 flratement of The advantage of this construction over the common camera 

 n» advantages, .Vj Scura j g such, that no one who makes the comparison, can 

 doubt of its superiority ; but the causes of this may require 

 some explanation. It has been already observed, that by the 

 common lens, any oblique pencil of rays is brought to a focut 

 at a distance less than that of the principal focus. But In the 

 Construction above described, the focal distance of oblique penr 

 cils is not merely as great, but is greater than that of a direct 

 pencil. For since the effect of the first surface is to occasion 

 divergence of parallel rays, and thereby to elongate the focus 

 ultimately produced by the second surface, and since the de- 

 gree of that divergence is increased by obliquity of incidence, 

 The oblique the focal length resulting from the combined action of both 



pencils have a surfaces will be greater than in the centre, if the incidence on 



longer focus ° . 



•than the prin- «ie second surface be not so oblique as to increase the con- 



tipai focus. \>ergence. On this account, the opening E is placed so much 

 nearer to the lens than the centre of its second surface, that 

 oblique rays Ef, after being refracted at the first surface, are 

 transmitted through the len3 nearly in the direction of its 

 shorter radius ; and Tience are made to converge to a point so 

 distant, that the image (at /) falls very nearly in the same plane 

 with that of an object centrally placed . 

 The aperture In the use of spectacles by long-sighted persons, the course 

 struction re- °*" tne ra y s * n $9- °PP 0S ' te direction is so precisely similar, 

 presents by that the same figure might serve to illustrate the advantages 

 oTpT^oTthe 6 °^ t " e periscopic construction. For the purpose of seeing the 

 eye in the p. extended page of a book (as at AB) with least fatigue to the 

 spectacles. e ^ t k a t form of lens will be most beneficial, which renders 

 the rays received from each part of its surface parallel ; and 

 this is effected by the exact counterpart to the preceding ar- 

 rangement 3 for in this case the opening E represents the 

 place of the eye receiving parallel rays from the lens in each 

 direction, instead of transmitting them from a distance towards 

 it. 

 fimit of ad- There is, however, this difference between the two cases, 



that 



