PBRISCOPIC CAMERA AND MICROSCOPE. 27 



I have therefore had recourse to experiments, in order to Experiment 

 determine by what construction the field of distinct represen- P referable - 

 tation may be most extended j and I trust the result will be 

 acceptable to this society. I shall take the same opportunity 

 to describe an improvement in the construction of the simple . - •_ 

 microscope, which may also be termed periscopic, as the ob- croscope. 

 ject 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, however, in 

 which this is effected is apparently somewhat different in the 

 practical execution. 



In the common camera obscura, where the images of distant In the corn- 

 objects are formed on a plane surface to which the lens is J^sideTmages 

 parallel, if the surfaces of the lens be both convex, and equally are indistinct, 

 curved (as in fig. 1,P1.I); and if the distance of the lens be such, 

 that the images formed in the direction of its axis CF be most 

 distinct, then the images of lateral objects are indistinct in a 

 greater or less degree, accordingly as they are more or less 

 remote from the axis. The causes of this indistinctness may because the 



be considered as twofold ; for in the first place, all parts of pl ane is m « re 

 Al , , ■. . . L ,1. distant than 



the plane, excepting the central point, are at a greater dis- t ^ e pr i nc j pa i 



tance from the centre of the lens, than its principal focus j and focus, 

 secondly, the point /, to which any pencil of parallel rays, q"i , qU e pen- 

 passing obliquely through the lens, are made to converge, is cils have a fo- 

 less distant than the prin«ipal focus. On this account, it is in £ us 8tlU short " 

 general best to place the lens at a distance somewhat less than 

 that which would give most distinctness to the central images, 

 because in that case a certain moderate extension is given to 

 the field of view 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 

 only diminished in degree, but is not remedied. 



The construction, by which I propose to obviate this defect, New construc- 



is represented in the second figure, in which are seen the essen- tion - Witn a 

 ,. , ~ .1.1 . meniscus lent 



tial parts or a periscopic camera in their due proportion to concave to- 

 each other. The lens is a meniscus, with the curvatures of wards the 

 its surfaces about in the proportion of two to one, so placed aperture at a* 

 that its concavity is presented to the objects, and its convexity distance from 

 toward the plane on which the images are formed. The f^ 01 "*™ 



aper- 



