CH. XI] OPTICS OF MOVING PICTURES 413 



even illumination of the film picture and the screen image if con- 

 densers were obtainable entirely free from spherical aberration, 

 but this is not practical. No rule can be given as to the best 

 position of the arc and condenser but the best position must be 

 determined for each particular case. Some general hints can, 

 however, be given. 



First The objective should be of large diameter. This will 

 allow of a greater range of adjustment through which good illumin- 

 ation can be obtained ( 829-830). 



The lenses of the double-lens condenser (fig. i) should be as near 

 together as possible without actually touching. The convex sides 

 of the condenser lenses should face each other, the plane sides 

 should face the lamp and the objective. 



The condenser lenses should be of fairly long focus, 18 to 19 

 cm. (7 to y>2 in.). 



If the condenser is as far away from the aperture plate as 

 possible the illumination is usually more even, though less intense 

 than when the condenser is close to the aperture plate. 



When first setting up the machine, it is a great help to have a 

 series of condenser lenses to try, say such a series as two lenses of 

 14 cm. focus, one each of 15, 16, 17 cm., two of 19 cm. focus, (two 

 of 5^" in., one each of 6, 6>^, 7 in., two of 7^ in.). The two con- 

 denser lenses should be of the same focus, then only one kind of 

 condenser lens will need to be kept in stock to supply breakage. 



When the adjustment for distance is to be made, move the lamp- 

 house with its condenser close to the aperture plate, fasten it 

 in position, move the arc in the lamp-house nearer to and farther 

 from the condenser until the best light is obtained on the screen. 

 Note how this light appears and whether there are any ghosts or 

 shadows. Then fasten the lamp-house and condenser slightly 

 farther from the aperture plate and move the arc until the best 

 light is again obtained. After repeating this, for every position 

 of the condenser, the condenser is set at the distance which was 

 found to be best. It may be necessary to try a different set 

 of condenser lenses before the best possible result is obtained. 

 This is a rather tedious process but is well worth while doing. 



