586 IMAGE FORMATION, INVERSION OP IMAGES [Cn. XIV 



If placed at b or e (fig. 326), it cuts off the margin of the cone and 

 lets the center through but if placed at g, light from the center and 

 the margin of the condenser gets through, but light from a zone part 

 way out is removed (fig. 327). 



The result on the illumination of 

 an object placed in the converging 

 cone of light will be as shown in fig. 

 3266. An object placed near the 

 condenser will be evenly illumina- 

 ted. As it is moved away from the 

 condenser face towards the crossing 

 of the rays the outer edge first be- 

 comes more brightly illuminated 

 than the center and then the spotted 

 effects shown in the figure will be 



FIG. 327. APPEARANCE ON THE se en. At no position will there be 

 SCREEN WHEN ILLUMINATED BY ... ,. . ,, , . 



THE CONDENSER SHOWN IN an even illumination of the object 



FIG. 326A IF DIAPHRAGM is when using a point source except 



HELD IN THE POSITION g. , , , , . , . , , 



when the object is placed next to the 



condenser face, a. If it is necessary to eliminate the spotted effect 

 due to spherical aberration as when exhibiting moving pictures one 

 must use an extended source of light, so that the aberration figures 

 from the different points of the source overlap. The arc lamp with 

 15 to 20 amperes direct current is sufficiently extended to give an 

 even illumination provided a short focus condenser is used. 



815. Spherical aberration of the condenser with the magic 

 lantern. When using the magic lantern the spherical aberration 

 of the condenser, unless exceedingly great, is of no special disad- 

 vantage. The rays from the different parts of the slide will not all 

 cross at the center of the objective but will cross at different points 

 on the axis (fig. 320, 337). If the objective is of good quality and 

 of large enough diameter to include all of the beam of light there 

 will result a good screen picture. 



816. Effect of an extended source. Let a, fig. 330, represent 

 a point in the slide S. Light which has come from all parts of the 



