CH. X] 



TROUBLES IN DRAWING 



387 



than for a slide which is thin. Attention to the substage con- 

 denser will make a great difference with one's success. 



(4) The right-angled arc lamp should be used in drawing 

 because if the microscope and lamp are properly arranged the source 

 of light will remain in the axis no matter how long the lamp burns. 

 If an inclined carbon lamp or one with both carbons in the vertical 

 or horizontal position is used the source of light is constantly 

 getting out of the axis from the burning away of the carbons, 

 consequently they must be fed up more frequently to keep the 

 source of light in the field. 



(5) The picture will be distorted unless the axial ray strikes the 

 drawing surface at right angles. Therefore, in using a prism or 

 mirror for a horizontal surface the microscope must be horizontal 

 and the mirror or prism at 45 degrees to reflect the axial ray ver- 

 tically downward. If the mirror or prism is twisted over to one 

 side the axial ray will not strike the surface at right angles and there 

 will be distortion. If one has a micrometer in squares it is easy to 

 determine whether the image is distorted or not. 



(6) The image will be erect only when the object is properly 

 placed on the stage. 



(7) If a glass mirror silvered on the back is used, and the object 

 is quite opaque the secondary image reflected from the face is 



FIG. 220. SLIDE OF SERIAL SECTIONS WITH -a, k- ON THE COVER-GLASS 



TO ENABLE ONE TO DETERMINE WHEN THE IMAGE ON THE DRAWING 



SURFACE is ERECT (See fig. 143, and 512, 517). 



