188 



THE GRAPHO- PRISM AND THE TECHNIQUE 



If only an artistic (impressional) reproduction is required, it 

 may generally be produced by freehand without the aid of any 

 accessories, since in such a case the mathematical correctness is 

 not so necessary as in a drawing made for scientific study, where 

 all depends upon its mathematical correctness if it is to be instruc- 

 tive, and under the latter instances the optical measurements are 

 the first consideration and of paramount importance. 



Fig. 22. —Abbe's Grapho- Prism. 

 The most practical and most frequently applied drawing-appa- 

 ratus in microscopical work is the Camera Lucida. This and all 

 similar instruments work upon the same principle — that is, the 

 object and the paper are seen with the one eye, while at the same 

 time the picture is reflected into the eye by means of the mirror 

 or prism. As the picture is seen upon the paper beside the micro- 

 scope, its contour can easily be reproduced upon the paper with 

 the point of a pencil. Thus a fac-simile of the utmost mathema- 

 tical and scientific correctness and exactness may readily be 

 produced. He who has by practice learned to look into the 

 microscope with one eye and to hold the other eye open at the 

 same time may succeed even without the use of a Camera Lucida. 

 If he gazes with one eye into the microscope and with the other 

 eye at a piece of paper lying beside it, in a few moments the 

 observer will find the object projected upon the paper, and will 

 thus be able to sketch the outlines of the image with comparative 

 ease and exactness. 



