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Rays connecting the conjugate points i, 2 and 3 of the source S and its image 6". From each of these points of the source a ray is drawn through center, upper and lower margin of the condenser C. The latter is represented here and in the two following diagrams by a pair of parallel lines.
connecting them are designated with the same figures (i), (2), (3).
Figure 10 illustrates how the ray from point I through the lower margin of the condenser does not reach the image point i and how the angle of useful radiation is smaller by the shaded portion than it would be were it not for the diaphragm action of the aperture plate.
Only the portions in the middle part of the source send forth cones which fill all the aperture of the condenser, while all the shaded parts of the pencils from points i and 3 are intercepted by the aperture plate as illustrated by the next drawing, figure 11.
We have to consider next the losses caused by
Absorption and reflection. The amount of light lost by absorption in a piece of glass depends upon the absorption coefficient of the material and on the thickness of the piece. The absorption coefficient varies from 1.3% per cm. for very clear glass to about 4% per cm. for very bad glass. The loss of light in a lens varies with the zones through which the light passes, depending on the different lengths of the path between the surfaces.
The loss by reflection depends upon the index of refraction of the
Fig. 10 The white circular areas at the right and left show the part of the image of the source imaged by the whole aperture of the condenser and the image of this part. The shaded anular zone around these circles represent the part of the radiation which is only partially by the condenser. The imperfect utilization of the area of the source is caused by the interposition of the aperture plate T.
S3