Bauhaus-Universität Weimar

Optic Projection: Principles, Installation and Use of the Magic Lantern, Projection Microscope, Reflecting Lantern, Moving Picture Machine
Gage, Henry Simon and Henry Phelps Gage
The illuminated aperture might be decreased by using a large 
iris diaphragm to cover part of the condenser face a b. 
In the figure the aperture illuminated a'b', is less than the 
diameter of the rear lens. If the size of the condenser were greatly 
increased until its image was as large as the rear lens of the objec¬ 
tive, the marginal ray from s, would move from sxa's' to szs’. 
The entire aperture of the objective would be illuminated and no 
more light would be used by a further increase in the size of the 
condenser (Fig. 347). 
§ 859. Image formation of a point not on the axis.—Light from 
/, will spread out over the angle w t y, which equals angle a t b, will 
pass through a' b\ and be collected to a point t', on the screen. 
This light will of course fill a cone of which the limiting rays are 
twb’t' and t y a’ V (Fig. 347). 
§ 860. Illumination of the screen image.—Any single point on 
the screen as s' or t', will be illuminated by light which has come 
from the bright disc a' b'. The illumination will therefore depend 
on the three factors, the brightness and area of the disc a'b', and 
its distance from the screen (Fig. 347). 
The area of the disc can, of course, be no greater than the area of 
the back lens of the objective, and is usually smaller. For this 
reason the brightest projection in a given case is obtained when the 
back lens of the objective appears to be entirely filled with light. 
The brightness of this disc of light would, if it were not for light 
losses, be exactly the same as that of the original source. This 
follows from the fact that the brightness of an object remains the 
same, except for light losses, when seen through a lens or a system 
of lenses as when viewed directly. A lens can only change the 
direction, not the intensity of light, or in other words it can only 
change the apparent size of an object. 
This being the case the screen brightness is limited not by the 
candle-power of a source but by its intrinsic brilliancy (candle- 
power per square centimeter). This assumes the image of the 
light to have an area great enough to cover the front lens of the 
objective, which is the case with most microscopic projection.


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