42 



III. PROTOPLASMIC MOVEMENTS MICRO. DRAWINGS 



The second camera (also by Zeiss) is known as the two-prism 

 camera, and is illustrated in Fig. 16, together with a diagram 

 showing the path of the light rays when it is in use. The prism- 

 sheath (K) is fixed upon an adjustable arm (b), and this is mounted 

 by a vertical arm upon a collar which slides under the eye-piece 

 on the end of the tube. This camera has the advantage that it 

 can always be kept on the instrument, the prism-sheath being 

 turned out of the way ; and with some practice will perform yeo- 

 man's service. It consists of two prisms, inclined to one another, 

 in a common setting. The rays (s) coming from the pencil take, 

 after double reflection inside the prisms, a course parallel to the 

 axis of the microscope, and thus coincide with the rays (BJ) 

 coming direct from the object. The camera is placed in the 

 inclination represented in the figure, and so adjusted that its 



Si S 



FIG. 16. Two- prism Drawing Camera by Zeiss ; three-quarters natural size. 



anterior edge, visible through the opening in the setting, approxi- 

 mately bisects the "pupil" of the emerging rays of the micro- 

 scope, i.e., the bright circular disk which we notice when we look 

 perpendicularly into the eye-piece from a short distance, such 

 inch, above it. If, then, on moving the head to one side 



as 



we do not see the "pupil" notably displaced towards the edge of 

 the prism, this latter stands also at the right height. We draw 

 upon a sloping drawing-desk, which is placed in front of the 

 microscope. If, after some attempts, we have found the point 

 of the lead pencil upon the drawing-paper, we can now follow 

 with it the outlines of the object. If the object is not to be 

 distorted in drawing, the drawing-desk must have a correct 

 inclination. In order to determine this rapidly and accurately,. 

 we draw the circular outline of the field of view upon the paper 



