6i8 EFFECT OF APERTURE IN PROJECTION [Cn. XIV 



The effect of light losses by reflection and by absorption is to 

 reduce the brilliancy of the bright disc a'b'. These losses are very 

 great, and as only a small amount of light is available anyway, that 

 is the reason we do not recommend the use of the substage con- 

 denser except in the special cases of high power demonstration, for 

 photography and for high power drawing, where fine details are of 

 more importance than brilliancy (see Ch. IX, X, 401, 477). 



A substage condenser will reduce the brilliancy of the disc to 

 70% of its former value, and our experience has been that the full 

 aperture of all but the highest power objectives (8, 6, 4, 2 mm. 

 equivalent focus 8o8a) can be entirely filled without its use. 



861. Appearance when one looks into the objective. If the 

 eye is held at 5' (a dark glass being of course held in front of the 

 eye or better yet held just before the front of the objective at s) 

 light will strike the pupil from all parts of the condenser image 

 a' b', the appearance being that of a bright disc of light. 



The larger this disc, the greater the aperture of the objective 

 illuminated. With low powers the entire aperture will be illum- 

 inated by the use of the large condenser alone. With high powers 

 only the central part of the back lens will appear bright. When 

 the bright disc spreads over the entire back lens the aperture of 

 the objective is fully illuminated and no further increase of light 

 is possible with a given source. 



As often happens, the back lens appears illuminated not with a 

 uniform bright disc but by a bright ring with a bright center 

 separated by a dark ring or crescent. This is due to the spherical 

 aberration of the condenser. 



862. Appearance when an amplifier or an ocular is used. 



An amplifier or an ocular will spread the light from the objective 

 over a larger area than before, of course decreasing the brightness 

 of the screen image. This effect could be foretold by looking 

 directly at the instrument from the screen for the bright disc of 

 light a'b' (fig. 347) will appear smaller when the ocular or amplifier 

 is in place. 



863. Limit of brightness with the projection microscope. 



The screen image with microscopic projection apparatus is not as 



