CH. IXl 



USE OF PROJECTION MICROSCOPE 



249 



USE OF THE PROJECTION MICROSCOPE 



379. Objectives in a revolving nose-piece. For most projec- 

 tion a battery of three objectives would be sufficient. These 

 should be: (i) a low power objective to show entire specimens 

 (one of 40 to 50 mm. focus is good) ; (2) an intermediate objective 

 of 16 to 18 mm. focus; and (3) a high power, that is, one of 10 to 4 

 mm. equivalent focus ( 355). 



10 



20 



FIG. 141. SIDE AND FRONT VIEWS OF THE CRATER AND CARBONS 



BURNING WITH 10 AND WITH 20 AMPERES OF DIRECT 



CURRENT (Natural size). 



This picture is to show the increase in size of the crater with the larger cur- 

 rent. (See also fig. 292-293). 



(In making the photographs, the lamp was burning with the amperage indi- 

 cated, and an instantaneous exposure was made with a diaphragm of F/32. 

 The current was then turned off and the carbons exposed 90 seconds with a 

 diaphragm of F/8. This brought out the carbons, and gives the appearance 

 gained by the eye when suitably screened and looking at the burning lamp.) 



