CH. XIV] EFFECT OF APERTURE IN PROJECTION 



619 



bright as could be obtained with the magic lantern. The reason 

 is the physical impossibility of crowding more than a limited 

 amount of light through the very small opening of a microscopic 

 objective. The objective can be illuminated so that light comes 

 from the entire area of the rear objective lens to form the screen 

 image. When this occurs this objective will act like a luminous 

 source having the same intrinsic brilliancy (except for losses) as the 

 original source, and having the area of the rear lens. The area of 

 this rear lens is fixed and cannot be increased. The intrinsic 

 brilliancy of a given source is fixed. Hence, only by using a 

 brighter source as changing from a lime light to the alternating 

 current arc, from the alternating current arc to the direct current 

 arc, or from the direct current arc to sunlight, or by reducing the 

 losses due to unnecessary complication of lenses and condensers 

 between the source and the screen, can the image brightness be 

 increased. When using sunlight one has reached the limit of 

 possibility for light brilliancy. 



KOEHLER'S METHOD OF ILLUMINATING A SPECIMEN FOR 



MICROSCOPIC PROJECTION 



864. The simple method of lighting a specimen for micro- 

 scopic projection by a condenser is, as stated above, to focus the 

 image of the crater upon the front of the objective (fig. 140). For 

 high powers this is practically the same as if the crater image were 

 focused upon the specimen. 



With the Kcehler method a substage condenser is used. The 

 large condenser forms an image of the crater on the diaphragm of 



FIG. 348. ORDINARY METHOD OF ILLUMINATING MICROSCOPIC SPECIMENS. 



(See 376, 833). 



