CH. XIV] IMAGE FORMATION WITH MOVING PICTURES 591 



In microscopic projection (Ch. IX) and in drawing with the 

 microscope (Ch. X) the three-lens condenser with its small spheri- 

 cal aberration is of great advantage. For micro-projection without 

 a substage condenser, the final plano-convex lens of the triple con- 

 denser should have a fociis of about 15-20 cm. (6-8 in.). This will 

 answer well for objectives as high as 4 mm. equivalent focus (/^ 

 in.). Where a substage condenser is used, the focus of the last 



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FIG. 331. TWO-LENS CONDENSER FOR PROJECTION. 



The condenser (Cond) is shown in connection with the lamp-house and right- 

 angle arc lamp. 



The first lens of the condenser (i) is of shorter focus (i. e., thicker) than the 

 second lens (2). The condenser is nearer the source of light (L) than the 

 principal focus of the first condenser lens, hence the light beam between the 

 condenser lenses is diverging. With this arrangement and the lenses close 

 together a wider beam of light can be utilized for projection than as if the 

 condenser were farther from the lamp (see fig. 343). (For a more complete 

 explanation of this figure see fig. 379.) 



plano-convex lens of the large condenser should be longer than 

 15 cm. One of 25-40 cm. (10-16 in.) is more satisfactory (See 

 Ch. IX, 402). 



822. Image formation with moving pictures. When moving 

 pictures are to be projected, the conditions to be met are not so 

 simple as with the magic lantern, and one must bear in mind the 

 actual requirements. 



823. Practical requirements. These requirements are : (The 

 figures for equivalent focus refer to an actual case). The moving 



