CH. IX] CENTERING FOR MICRO-PROJECTION 247 



objectives a slight variation from perfect alignment would do no 

 particular harm, but the lenses arc so small in micro-projection 

 objectives that a very slight displacement from the axis would 

 throw the light outside the objective and spoil the projection. 



The fundamental principles and precise directions for centering 

 projection apparatus are given in Ch. I. 51-58. 



375. Final centering of the projection objective. After the 

 lamp and condenser are centered as nearly as possible and are at the 

 right distance apart ( 55, 56, 376), move the stage up toward the 

 condenser so that there is plenty of room between it and the objec- 

 tive. Use some dust or smoke to find where the cone of light from 

 the condenser comes to a focus (fig. 132, 323). 



Now move the microscope on its mounting toward the condenser. 

 If the objective is centered, then the point of light at the focus will 

 enter the front lens through the hole in the objective hood (fig. 140). 

 If it is not centered then it will appear at one side or even entirely 

 outside the objective. Use the fine adjusting screws of the arc 

 lamp and change the position of the image of the crater sufficiently 

 to direct the cone of light into the front lens of the objective. In 

 case the objective is greatly out of center it may be found necessary 

 to change the position of the entire microscope. 



376. Distance of the objective from the condenser. The 



objective should be at a distance which will bring the crossing point 

 of the rays in the cone from the condenser within the objective, as 

 for the magic lantern objective (fig. 122). As the center of the 

 objective is but slightly beyond the front lens, the following method 

 has been found to give excellent results. The objective is drawn 

 up toward the condenser until the image of the crater is shown 

 within the opening upon the black hood in front of the objective 

 (fig. 140). As the image is inverted the lower or negative carbon 

 will appear above in the image. If now the stage with a specimen 

 is moved up toward the objective until the microscopic object on 

 the stage is in focus, the image on the screen will be very brilliant. 

 One should make slight adjustments toward and away from the 

 condenser to get the most brilliant image. It will be found that 



