CH. ir\ 



DRAWING WITH THE MICROSCOPE 



145 



§ 203. Arrangement of the Camera Lucida Prism. — In 

 placing this camera lucida over the ocular for drawing or the deter- 

 mination of magnification, the center of the hole in the silvered 

 surface is placed in the optic axis of the microscope. This is done 

 by properly arranging the centering screws that clamp the camera 

 to the microscope tube or ocular. The perforation in the silvered 

 surface must also be at the level of the eye-point. In other words 

 the prism must be so arranged vertically and horizontally that the 

 hole in the silvered surface is in the axis of the microscope and coin- 



Fig. 12S. Abbe Camera Lucida in Position— -The 

 prism over the ocular may be turned aside for direct 

 observation. The light modifiers for drawing surface 

 and microscope are in connection with the prism. The 

 prism has centering screws and may be moved up or 

 down with the whole apparatus by the clamping ring 

 around the top of the draw-tube. This serves to place the prism at the proper 

 vertical level for the eye-point of different oculars. (Cut loaned by the Spen- 

 cer Lens Co.) 



cident with the eye-point of the ocular. If it is above or below, or 

 to one side of the eye.point, part or all of the field of the microscope 

 will be cut off. As stated above, the centering screws are for the 

 proper horizontal arrangement of the prism. The prism is set at 

 the right height by the makers for the eye-point of a medium ocular. 

 If one desires to use an ocular with the eye-point farther away or 

 nearer, as in using high or low oculars the position of the eye-point 

 may be determined as directed in § 67 and the prism loosened and 



