Dr. Wollaston on a 
37 s 
and applying their plane surfaces on opposite sides of the 
same aperture in a thin piece of metal (as is represented by a 
section, fig. 4), I produced the desired effect ; having virtually 
a double convex lens so contrived, that the passage of oblique 
pencils was at right angles with its surfaces, as well as the 
central pencil. With a lens so constructed, the perforation 
that appeared to give the most perfect distinctness was about 
one-fifth part of the focal length in diameter; and when such 
an aperture is well centered, the visible field is at least as 
much as twenty degrees in diameter. It is true, that a portion 
of light is lost by doubling the number of surfaces ; but this is 
more than compensated by the greater aperture, which, under 
these circumstances, is compatible with distinct vision. 
Beside the foregoing instances of the adaptation of peri- 
scopic principles, I should not omit to notice their application 
to the camera lucida; as there is one variety in its form, that 
was not noticed in the description which I originally gave of 
that instrument.* 
In drawing, by means of the camera lucida, distant objects 
are seen by rays twice reflected ( d , fig. 5), at the same time 
and in the same direction that rays (^) are received from the 
paper and pencil by the naked eye. The two reflections are 
effected in the interior of a four-sided glass prism, at two pos- 
terior surfaces inclined to each other at an angle of 135 de- 
grees. In the construction formerly described, the two other 
surfaces of the prism are both plane, through which the rays 
are simply transmitted at their entrance and exit. But since 
an eye that is adjusted for seeing the paper and pencil, which 
are at a short distance, cannot see more distant objects dis- 
* Nicholson’s Journal, XVII. p. i. Phil. Magaz. XXVII. p. 343^ 
