Camera Lucida . 
®7l 
parallel, by placing a lens at the distance of its principal 
focus from the paper, because the rays received from the 
distant objects are parallel ; so also when the object seen 
by reflection is at so short a distance that the rays receiv- 
ed from it are in a certain degree divergent, the rays from 
the paper should be made to have the same degree of di- 
vergency in order that the paper may be seen distinctly 
by the same eye ; and for this purpose the lens must be 
placed at a distance less than its principal focus. The 
stem of the instrument is accordingly marked at certain 
distances to which the conjugate foci are in the several 
proportions of 2, 3, % &c. to 1, so that distinct vision may 
be obtained in all cases, by placing the painting propor 
tionally more distant. 
By transposing the convex lens to the front of the 
instrument and reversing the proportional distances, 
the artist might also enlarge his smaller sketches with 
every desirable degree of correctness, and the natura 
list might delineate minute objects in any degree magni 
fled. 
Since the primary intention of this instrument is 
already, in some measure, answered by the Camera Oh- 
scura, a comparison will naturally be made between 
them. 
The objections to the Camera Obscura are 
1st. That it is too large to be carried about with con- 
venience. 
The Camera Lucida is as small and portable as can be 
wished. 
Sdly. In the former, all objects that are not situated 
near the centre of view are. more or less distorted. 
In this, there is no distortion ; so that every line, even 
the most remote from the centre of view, is as strait as 
those through the centre. 
