Dr. Wollaston on a periscopic Camera Obscura , &c. 371 
to describe an improvement in the construction of the simple 
microscope, which may also be termed periscopic, as the ob- 
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 
objects are formed on a plane surface to which the lens is 
parallel, if the surfaces of the lens be both convex, and equally 
curved (as in fig. 1) ; 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 
be considered as twofold ; for in the first place, all parts of 
the plane, excepting the central point, are at a greater dis- 
tance from the centre of the lens than its principal focus ; and 
secondly, the point /, to which any pencil of parallel rays 
passing obliquely through the lens are made to converge, is 
less distant than the principal 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 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, 
