IN TROD UCTION. 1 9 



tance that the image of one division of the stage 

 micrometer covers on the rule. If, for instance, a tenth 

 of a millimeter of the stage micrometer covers five 

 centimeters (five hundred tenths of a millimeter), any 

 drawing under the same adjustment will be magnified 

 five hundred times. -Always mark the number of 

 times magnified at the side of the drawing as in gross 

 anatomy, thus, x 500. 



The distance from the drawing-paper to the reflecting 

 surface of the camera lucida should be about the same 

 as from the latter to the outer lens of the object glass, 

 in order that the drawing may properly represent the 

 magnifying power of the instrument. Ten inches has 

 been adopted as the standard length of tube. 



Trace the image first with a hard pencil, and then 

 go over it with ink before the object is removed from 

 the instrument in order to correct any errors made by 

 the pencil. 



It is not an easy matter to draw accurately with the 

 camera lucida, owing to the difficulty in seeing both 

 the image and the pencil point distinctly at the same 

 time. Much depends on the relative amount of light 

 received by the eye from the instrument and from the 

 drawing-paper. If the pencil point does not show 

 clearly, there should be more light on the paper, and 

 if the image is not clear, more on the object. 



Invariably accompany each drawing with a full expla- 

 nation. 



IX. BOOKS OF REFERENCE. 



It should be the aim of the student to find out all 

 that he can about the plant in hand with as little assist- 



