Chap. XXVIII.] 



CAMERA Luc IDA. 



Here the change of eye-piece is all that is necessary 



to convert a monocular into a binocular, or to reverse 



the process. In all forms of 



the stereoscopic microscope, 



however, the loss of light, 



owing to so many reflecting 



surfaces, is so considerable, 



that for ordinary practical use 



the monocular microscope is 



the most serviceable. 



THE DRAWING OF MICRO- 

 SCOPIC OBJECTS. 



Various forms of optical Fig> lee.-Hartnack's Bino 

 apparatus have been devised for cuiar Eye-piece, 



fitting to a microscope, in order 



to permit of a faithful drawing being taken of the 

 magnified image. 



Wollaston's camera liirida, devised in 

 1807, is one form very generally employed. It con- 

 sists of a prism of glass set in a 

 brass case fixed to a short tube 

 which is slipped on the eye-piece 

 instead of its eye-glass. The body 

 of the microscope must be placed 

 horizontally. Fig. 167 represents 

 the path of the rays. 

 Rays of light oo, passing 

 up the microscope tube, 

 fall upon the perpendicular 

 face of the prism which 

 is next to the tube. They 

 meet this face at right 

 Fig. 167. Camera Lucida. angles, and pass unaffected 



into the prism, to fall 



on the lower internal face, where, owing to the 

 angle, they are totally reflected in an upward direction. 



