28o 



USE OF THE MICROSCOPE 



dividing i by .0225, which would give 44.4. Then the number 

 of stomata in one square millimeter would be 9 times 



44-4 = 399-6- 



The other method of drawing to scale is carried out with a 

 camera lucida, the most convenient form of which is shown in 

 Fig. 151. The main structural details of this instrument are, 

 behind the opening P a prism silvered on one of its surfaces 

 excepting for a narrow circular area at the center; and the plane 



Fig. 151.— TCamera lucida. M, l^irror; F, opening to reflecting prism; K, knobs for regu- 

 lating diaphragms that govern illumination from object and drawing paper. 



mirror at M. Through the unsilvered part of the prism one can 

 look and see the object; the mirror reflects the drawing paper 

 placed on the table below it to the silvered surface of the prism 

 and this reflects it into the eye. In this way the object and the 

 drawing paper and the pencil held over the drawing paper 

 are all seen at once superimposed; the object appears spread out 

 over the paper and with the pencil its outlines can easily be 

 traced. To use the camera lucida successfully it is necessary to 

 have some means of illuminating the object and drawing paper 

 with equal intensity, for if one appears brighter than the other 

 they cannot both be seen with equal clearness. If the objecj, for 

 instance, is too bright the point of the pencil cannot be accurately 

 followed, and if the paper has the stronger illumination the 



