6i6 



EFFECT OF APERTURE IN PROJECTION [Cn. XIV 



The light from the point 5, spreads out over the angle xsy, which 

 equals angle asb. Light from s, which has come from a, can reach 

 the screen along the path s x a' s'. From 6, the light follows the 

 path syb's', and from the central parts of the condenser light will 

 go from 5 to s', along the paths which lie between a' and b f . 



The result is that the light which goes to make up the point s', 

 of the screen image has come from the entire area of the circle a'b'. 

 That is, the circle a'b', is the diaphragm which limits the illum- 

 inated aperture of the objective. 



Fn;. 347. IMAC.E FORMATION WITH A MICROSCOPIC OBJECTIVE. 



The- shaded portion shows the cone of light which illuminates the point t, of 

 the specimen and which goes to make its image /', on the screen; all other 

 points are similarly lighted and similarly pass on to form the screen image. 



a,b The last element of the condenser (see fig. 332). 



5, / Two points of the specimen to be projected. 



O,, O 2 The front and back combinations of the objective. 



iv x y z Points on the front lens of the objective. 



a', b' Image of the condenser face. 



s', t' The inverted screen image of the object s, t. 



When used with a magic lantern, the projection objective transmits only 

 70% of the light reaching it. As only 68% of the original light reaches the 

 objective, the screen image must be formed by (68 x 70 = 47.6%) of the 

 original light. 



With the microscope, however, only about 6% of the original light gets 

 through the objective and goes on to form the screen image (fig. 342). If a 

 substage condenser and an ocular are used, the light for the screen image is still 

 further reduced. 



The illuminated aperture of the objective may be increased by : 



1. Using a larger diameter condenser of the same focal length. 



2. Using a shorter focus condenser of the same diameter. 

 Either method will increase the angle asb, and the diameter of 



the disc a'b'. 



