40 THE MICROSCOPIST. 



by a black metal ring. 'A paper placed beneath is visible 

 through the opening in the ring, and the image reflected 

 by the prism upon it can be traced by a pencil. It is neces- 

 sary to regulate the light so that the point of the pencil 

 may be seen. 



Dr. Beale has recommended, in lieu of the camera lucida, 

 a piece of slightly tinted plate glass (Fig. 18), placed in a 

 short tube over the eye-piece at an angle of 45. This is 

 a cheap and effective plan. A similar purpose is served 



Fio. 19. 



Beale's Tint-glass Camera. Sceminering's Steel Disk. 



by a little steel disk, smaller than the pupil of the eye, 

 placed at the same angle (Fig. 19). 



The most simple method of measuring objects is to 

 employ one of the above drawing instruments, placing 

 first on the microscope stage an ordinary micrometer, and 

 tracing its lines on the paper. Then the outline of the 

 object can be traced and compared with the lines. The 

 magnifying power of an object-glass can also be readily 

 found by throwing the image of the lines in a stage 

 micrometer upon a rule held ten inches below the eye- 

 piece, looking at the magnified image with one eye and 

 at the rule with the other. Dr. Beale strongly urges 

 observers to delineate their own work on wood or stone, 

 since they can do it more exactly and truthfully than the 



