5Q6 IMAGE FORMATION WITH MOVING PICTURES [Cn. XIV 



be dim but fairly uniform. The zones from c to d, and e to f, light 

 a narrow ring of the film near 5 and t, i. e,, a dim center and a bright 

 outside ring would be produced by the zone from c to/ (fig. 326, 

 position c). The zone b to c, lights the part of the film between 5 

 and r, and / to g, lights the part between t and r, the addition of 

 these zones is to increase the illumination of the center, making the 

 illumination more uniform. The narrow zones a-b, and g-h, out- 

 side this, further illuminate the region in the center of the film. It 

 is necessary to remark that with an actual point source the illum- 

 ination with this arrangement can never be really uniform but the 

 "aberration figure" will consist of a bright ring 5 t, a bright point r, 



r 



FIG. 337. CONVEX LENS SHOWING CHROMATIC ABERRATION. 



(From The Microscope) 



The ray of white light (w) is represented as dividing into the short waved, 

 blue (b) and the long waved, red (r) light. The blue (b) ray comes to a focus 

 nearer the lens and the red ray (r) farther from the lens than the principal 

 focus (/). Principal focus (/) for rays very near the axis; /' and/", foci of blue 

 and red light coming from near the edge of the lens. The intermediate wave 

 lengths would have foci all the way between /' and /". 



at the center (white ghost) and between will be a more or less 

 evenly lighted disc. When a slightly extended source is used how- 

 ever, the aberration figures for the different points of the source will 

 overlap and if the dimensions of the crater image are about one- 

 third as great as the aberration figure an even illumination may be 

 secured. 



829. Effect of the diameter of the objective. If for any 

 reason, as the insufficient diameter of the objective lenses, some 

 of the light rays are lost after passing the film, the effect on the 

 screen image is the same as if these rays never reached the film. 

 Thus, if the objective O, fig. 333, has such a small diameter that it 

 would not admit the ray b s w, the effect would be the same as if no 

 light reached s, from the point, b, of the condenser. 



