268 
Camera Lacida . 
they may be discerned in this manner sufficiently well for 
determining the principal positions. 
The pencil, however, and any object, which it is to 
trace, cannot both be seen distinctly in the same state of 
the eye, on account of the difference of their distances, 
and the efforts of successive adaption of the eye to one or 
to the other, would become painful if frequently repeat- 
ed. In order to remedy this inconvenience, the paper 
and pencil may be viewed through a convex lens of 
such a focus, as to require no more effort than is neces- 
sary for seeing the distant objects distinctly. These will 
then appear to correspond with the paper in distance as 
well as direction , and may be drawn with facility, and 
with any desired degree of precision. 
This arrangement of glasses will probably be best un- 
derstood from inspection of fig. i, ah in the transparent 
glass ; h c the lower reflector ; h d a convex lens (of twelve 
inches focus) e the position of the eye; and / g h e the 
course of the rays. See plate 8. 
In some cases a different construction will be prefera- 
ble. Those eyes, which without assistance are adapted 
to seeing near objects alone, will not admit of the use of 
a convex glass ; but will on the contrary require one that 
is concave to be placed in front, to render the distant ob- 
jects distinct. The frame for a glass of this construc- 
tion is represented at i Jc, (fig. 3,) turning upon the same 
hinge at h with a convex glass in the frame l m , and 
moving in such a manner, that either of the glasses may 
be turned alone into its place, as may be necessary to suit 
an eye that is long or short sighted. Those persons, 
however, whose sight is nearly perfect, may at pleasure 
use either of the glasses. 
The instrument represented in that figure differs more- 
over in other respects from the foregoing, which I have 
chosen to describe first, because the action of the reflec- 
