CH. XIV] EFFECT OF APERTURE IN PROJECTION 



^\\ urn mated by 



FIG. 346. 



CANDLE-POWER OF A SURFACE ILLUMINATED AT 

 A GIVEN INTENSITY. 



O A surface having an area of one square centimeter located at the center 

 of a hemisphere of one meter radius. This surface is illuminated at an inten- 

 sity of / meter candles. It receives and reflects 1/10,000 lumens. As a light 

 source it has in the direction OP, B candle-power. 



OP Perpendicular to the surface. 



b'a' ab A zone on the surface of the hemisphere. This zone is located at 

 an angular distance of 0, from the perpendicular to the surface. The angle 

 subtended by this zone from the center is d0, and its width ab, is d0 meters. 

 The radius aN, of the circle aa 1 , is sin 9 meters, and its circumference is 2ir sin 0. 

 The area of the zone is then 2 sin dO. 



The intensity of illumination of this zone is B cos 0, meter candles. The 

 light flux received by this zone is then (illumination x area) equal to 

 B cos x 2 TT sin d = 2 IT B cos. sin 6 d 0. 



IMAGE FORMATION WITH THE MICROSCOPIC OBJECTIVE WITH 

 REFERENCE TO APERTURE 



858. Let a b, fig. 347, represent the face of the condenser 

 which is in such a position with respect to the objective that its 

 image s' t', is in focus on the screen. With high powers the speci- 

 men will be very close to the front of the objective. 



The front lens or combination of the objective 0\, will form an 

 image a' b', of the condenser face which may or may not coincide 

 with the back lens 0%, of the objective as here shown. 



Tracing the light from the condenser we see that all the light 

 from a, which gets through the front lens passes through a', and all 

 light from b, passes through b', and so on. 



