266 



THE AMERICAN MONTHLY [December, 



axis so as to bring the reflecting surfoces at different angles. The pur- 

 pose of a drawing prism is obvious as soon as it has been placed upon 

 the ocular and adjusted. 



Oberhauser's camera (Fig. 6) is somewhat more complicated than the 

 others. For this reason I will give more in particular the details in re- 

 gard to its practical application. The ascending rays from the objec- 

 tive are totally reflected through the large prism {d) into the horizontal 

 arm {A). If the ocular is placed in a horizontal position {B) it di- 

 rects the rays into the small prism ( C) upon the figure of an angle of 

 45° if focused in the right position, where it is again reflected at a right 

 angle into the observing eye. 



Fig. 6 — Oberhauser's Grapho-prism. 



Oberhauser's camera is very much liked for this reason : it does not 

 create a disturbance nor a confusion by the reflection of the picture at 

 a right angle upon the projected paper placed in a horizontal position. 

 The Oberhauser camera is attached to the tube of the microscope at 

 the ocular end without any trouble or loss of time. With but one 

 exception this camera is perfect ; it has a deficiency in one particular. 

 When the microscopic picture is twice reflected it then loses consider- 

 able in accuracy, that is, in its clearness and exactness. This is especi- 

 ally so in the use of higher powers, oil immersions, etc. By the most 

 concentrated light only can the special and superficial contour of the 

 microscopic picture be best produced. 



The practical and most applied drawing apparatus in microscopical 

 work is the camera lucida. The object and the paper is seen with 

 the one eye, and 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 microscope, its contour can be reproduced upon the 

 paper with the point of a pencil, and that, too, witli mathematical and 

 scientific exactness ; but he who has by practice learned to look into 



