338 DRAWING WITH HIGH POWERS [Cn. X 



If a mirror is used it must be a perfect one and preferably slivered 

 on the face to avoid duplicating the images. If it is silvered on the 

 back the glass must be thin. A totally reflecting prism is best, 

 but it is somewhat expensive, costing about twice as much as the 

 mirror. 



483. Avoidance of distortion. Whichever is used for reflect- 

 ing, it should be fitted with a stop so that it will be at 45 degrees 

 with the main axis, then the image-forming rays will be reflected 

 directly downward and the image will not be distorted, provided 

 of course, that the mirror or prism is directly above the drawing 

 surface. If it were turned over to one side more or less, the image 

 would be correspondingly distorted. 



It is a good plan for one to become familiar with the distortions 

 possible in drawing. For example, if the mirror or prism is not at 

 45 degrees with a horizontal microscope (fig. 182, 193), the spot of 

 light on the drawing surface will not be circular but elliptic, the axis 

 of the ellipse being parallel with the optic axis of the microscope. 

 If the prism or mirror is not directly above, but turned to one side, 

 then the spot of light will be elliptic and projected to one side of the 

 axis of the microscope. If one is familiar with the possible dis- 

 tortions it will be easy to detect them ; then they can be corrected. 

 Naturally, a drawing should be accurate when finished. 



484. Specimens suitable for drawing with high powers. Any 



object suitable for projection can have its image projected upon a 

 drawing surface (see also 410). 



485. Amount of electric current required for drawing. If one 

 has a direct current, 5 to 10 amperes will be sufficient for all draw- 

 ing purposes. The specimens must usually be left for a consider- 

 able time in the focus or near the focus of the light beam, and hence 

 are liable to overheating. The lower the amperage the less the 

 danger from the overheating. Then it is not good for the eyes 

 of the artist to have the light on the drawing surface too dazzling. 



With alternating current, 6 to 15 amperes usually suffice. 



Here, as in all other projection, skill is of more account than 

 overwhelming electric currents. 



