CH V] 



DRA WING WITH THE MICROSCOPE 



125 



upper one. A small oval hole is then cut out of the center of the sil- 

 vered surface and the two prisms are cemented together in the form of 

 the original cube with a perforated 45 degree mirror within it (Fig. 

 109, ab). The upper surface of the cube is covered by a perforated 

 metal plate. This cube is placed over the ocular in such a way that 

 the light from the microscope passes through the hole in the silvered 

 face and thence directly to the eye. Light from the drawing surface 

 is reflected by a mirror to the silvered surface of the prism and reflect- 

 ed by this surface to the eye in 

 company with the rays from the 

 microscope, so that the two fields 

 appear as one, and the image is 

 seen as if on the drawing surface 

 (Figs. 109, 114). It is designed 

 for use with a vertical micro- 

 scope, but see § 184. 



Fig. 112. Wollaslon's Camera 

 Lucida, showing the rays from the 

 microscope and from the drawing sttr- 

 face, and the position of the pupil of 

 the eye. 



For full explanation see Fig. 97. 



§ 182. Arrangement of the Camera Lucida Prism. — In plac- 

 ing this camera lucida over the ocular for drawing or the determination 

 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 arrang- 

 ing 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 will be 

 in the axis of the microscope and coincident 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 o\ low 

 oculars the position of the eye-point may be determined as directd in 



