CH. XIV] IMAGE FORMATION, INVERSION OF IMAGES 



585 



813. Actual case. The actual case differs from the ideal case 

 in that the condenser has a considerable amount of spherical aber- 

 ration and that the source is not a point but is somewhat extended. 



814. Spherical aberration of the condenser. The effect of 

 the spherical aberration of the condenser has not been sufficiently 

 studied up to the present, but it exerts a good deal of influence in 

 projection especially with micro-projection and with moving 

 pictures. 



FIG. 326A. THE PATH OF THE LIGHT RAYS FROM THE DIFFERENT 

 ZONES OF THE CONDENSER. 



FIG. 3266. THE APPEARANCES ON A CARD HELD IN THE DIFFERENT PARTS 

 OF THE CONE OF LIGHT. 



a, b, c, d, e, f Lines showing where the card was held in the light cone to 

 give the appearances in B. 

 g Diaphragm. 



As was shown in 809, the light from a point source will not come 

 to a focus in a point, but the rays passing through the margin of the 

 lens will be relatively more bent and will cross the axis sooner than 

 those which pass through the lens near the axis (fig. 320-323, 337). 



A curious phenomenon is the effect on the illumination of the 

 screen when a diaphragm is used to cut off the margin of the cone. 



