THE CAMERA LUCIDA. 



looks into it, and sees the object to which it is directed, and at the 

 same time sees, in the same direction, the sheet of paper which is 

 upon his table, so that in fact, the object to be drawn, or its 

 optical image, is seen projected and depicted on the paper. If he 

 take in his hand a pencil, and direct it to the paper, as if he were 

 about to write or draw with it, he will see his own hand and the 

 pencil directed to the paper upon which the object is already 

 optically delineated ; and he will consequently be able, with the 

 utmost facility and precision, to conduct the point of the pencil 

 over the outlines of the object and those of every part of it, so as 

 to make as correct a drawing of it as could be made by the process 

 of tracing, in which a picture, placed under semi-transparent 

 paper is traced by a pencil moving over its outlines. 



4. To present the principle of this contrivance under its most 

 simple point of view, let A B, fig. 1, be an object which would be 



Fig. 1. 



seen by the eye of an observer at E, under the visual angle A E B, 

 and let p r, be a sheet of paper, placed upon a horizontal table 

 before the observer. Now let a piece of plane glass, one half of 

 which is silvered on the lower surface, be placed at an angle of 45, 

 with the direction in which the object A B is seen, so as to intercept 

 the view of it from the eye at E ; the rays of A E and B E, which 

 encounter the silvered part of the glass, and which previously pro- 

 ceeded to E, will now be reflected to o, still, however, retaining the 

 same divergence, so that they will enter the eye E' of the observer, 

 186 



