and Microscope. 125 



opportunity to describe an improvement in the construction 

 of the simple microscope, which may also be termed peri- 

 scopic, as the object of it is to gain an extension of the 

 field of view, upon the same principles as in the preceding 

 instances, namely, by occasioning all pencils to pass as nearly 

 as may be at right angles to the surfaces of the lens. The 

 mode,' howeveT, in which this is effected is apparently 

 somewhat different in the practical execution. 



In the common camera obscura, where the images of 

 distant objects are formed on a plane surface to which the 

 lens is p:irallel, if the surfaces of the lens be ho^h convex, 

 and equally curved (as m tig. 1. Plate IV) ; and if the distance 

 of the lens' be such, that the images formed in the direction 

 of its axis CF be most distinct, tlfen the images of lateral ob- 

 jects are indistinct in a greater or less degree, accordingly 

 as they are more or less remote from the axis. The causes 

 of this indistinctness maybe considered as twofold ; for, in 

 the first place, all parts of the plane, excepting the central 

 point, are at a greater distance from the centre of the lens 

 than its principal focus ; and secondly, the pomt f, to 

 which any pencil of parallel rays passing obliquely through 

 the lens are made to converge, is less distant than the prin- 

 cipal focus. On this account, it is in general best to place 

 the lens at a distance somewhat less than that which would 

 give most distinctness to the central im;iges, because in Miat 

 case ascertain moderate extension is given to the field of 

 )riew, from an adjustment better adapted to lateral objects, 

 without materially impairing the brightness of those in the 

 centre. The want of distinctness, however, is even then, 

 onlv diminished in degree, but is not remedied. 



The construction by which I propose to obviate this de- 

 fect is represented in tl:e second figure, in which are seen 

 the essential parts of a periscopic camera in their due pro- 

 portion to each other. The lens is a meniscus, with the 

 curvatures of its surfaces about in the proportion of two to 

 one, so placed that its concavity is prcsenteil to the objects, 

 and its convexity toward the j)lane on which the iniagco 

 are formed. 1"hc aperture of the lens is four inches, it* 

 focus about twenty-two. There is also u circular openmg, 

 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. 



The advantage of this construction over the common ca- 

 mera obscura is such, that no one who makes the com- 

 parison can doubt of its superiority j but the causes of this 



mav 



