Volltext: Optic Projection: Principles, Installation and Use of the Magic Lantern, Projection Microscope, Reflecting Lantern, Moving Picture Machine

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 § 808a) can be entirely filled without its use. 
§ 861. Appearance when one looks into the objective.—If the 
eye is held at s' (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


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