DRAWING CAMERAS 129 



the image-forming rays from the drawing paper BB reflected by 

 the mirror M to the reflecting surface ef of the Abbe prism P, 

 and thence to the eye of the observer. The solid lines indicate 

 the image-forming rays from the preparation upon the stage of the 

 microscope, passing through the aperture in ef also reaching 

 the eye. It is obvious that the observer is able to see both the 

 image of the preparation and the drawing paper and can there- 

 fore trace upon the paper with a pencil the outlines and many 

 details of structure of the preparation. 



In order to avoid distortion of the drawing the mirror M must 

 be so inclined that the light ray be shall fall normal to the paper. 



From an examination of the diagram it will be seen that unless 

 the opening in ef is placed at the eye-point considerable light will 

 be lost and unsatisfactory results will be obtained. Before at- 

 taching a drawing camera always first ascertain the position of 

 the eye-point (see page 13). It not infrequently happens that in 

 designing an ocular, the manufacturer fails to take into account 

 the fact that the investigator may wish to use a drawing camera. 

 The eye-point may in such cases lie so close to the eye lens or 

 may lie so far above it as to render the employment of an Abbe 

 prism camera impracticable. Because of this great difference in 

 the relative position of the eye-point in different oculars it is best, 

 in purchasing an Abbe camera, to select one of the type shown 

 in Fig. 63, since in instruments of this sort the prism mounting 

 is of the smallest dimensions possible and the distance between 

 prism and clamping ring will allow exceedingly great latitude in 

 movements up and down. 



In order to equalize the light intensity reaching the eye from 

 preparation and drawing paper, a series of dark glasses of graded 

 degrees are mounted so as to turn and be swung in position, by a 

 ring between prism and paper, and a ring between prism and 

 ocular. By properly adjusting the diaphragm of the Abbe con- 

 denser and then selecting the right glasses in these rings, it is 

 always possible to obtain a clear image of both preparation and 

 drawing pencil. 



The large cameras of the type just referred to, are provided 

 with a graduated extension bar to which the mirror is attached 



