87 * 
Dr. Wollaston on a 
is represented in the second figure, in which are seen the 
essentia] parts of a periscopic camera in their due proportion 
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 presented to the objects, and its convexity 
toward the plane on which the images are formed. The 
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. 
The advantage of this construction over the common camera 
obscura is 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 focus 
at a distance less than that of the principal focus. But in the 
construction above described, the focal distance of oblique pen- 
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 focal length resulting from the combined action of both 
surfaces will be greater than in the centre, if the incidence on 
the second surface be not so oblique as to increase the con- 
vergence. 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 lens nearly in the direction of its 
