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Projection by means of a point shaped source and a spherically uncorrected condenser. The rays going through the margin of the condenser intersect the axis at S' margin nearer the lens and under a greater angle than those going through the lens near the axis which intersect the axis at S' axis. Parts' nearer the margin of the object are imaged with higher magnification than those near the axis with the result that the picture on the screen shows cushion shaped distortion.
radiation than without it, we must be aware of the fact that the greatest part of the radiation from the source is lost as indicated in figure 2, because the radiation within the solid angle intercepted by the condenser is only a small portion of the total amount, which the source sends out in all directions. For optical reasons it is impossible to increase the diameter of the condenser very much beyond a size which takes in an angle of about 90 . The radiation within this angle can be increased considerably by placing a concave mirror behind the source so that its center of curvature lies in the source, figure 2. Assuming no loss of light by reflection on the mirror and by passing through the source the quantity of light reaching the condenser should be doubled by this expedient.
The conditions so far assumed do not exist in practice, because neither the condenser is optically perfect nor the source point shaped. We shall consider an
(a) Imperfect condenser in combination with a point shaped source, and understand by term imperfect nothing due to careless making of the condenser, but those imperfections in performance which are due to the nature of a simple lens system. These are spherical and chromatic aberrations. On account of the former, the different zones of the condenser will produce a series of images of the source; the nearer to the margin the zone, the nearer to the condenser the image produced by it, while with a spherically corrected condenser the rays from all the different zones of the condenser go through the same
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