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point, or the hollow cones of light formed by the circumference of the zone as base line have the same apices. These cones have different apices with a spherically uncorrected condenser, the apices lying the nearer the condenser lens, the greater the diameter of the zone. The rays which pass through the margin of the condenser will, therefore, intersect the axis at 2: figure 3, under a greater angle than the corresponding rays in the arrangement with the perfect condenser. This increase of the angles will be small for rays passing near the center of the condenser and will become the greater the nearer to the margin of the lens the zone lies which produces the image. The effect is that the marginal parts of the field are imaged under a greater magnification than the center of the field. The image of a square object will show what is termed as cushion shaped distortion.
On account of the chromatic aberration, an uncorrected lens will produce images for the different colors composing the white light at the source at different distances from the condenser, the blue image
^SCREEN BLUE RED
RED BLUE
. . . ^^g 4
Projection of a transparent point P of a stencil T by a chromatically not corrected
condenser. The image of the source formed by the blue component of a white ray coming from the source and going through P lies nearer to the lens, at S' Blue than the image formed at S' Red by the red component of another white ray coming from the source. Therefore the blue image lies farther away from the center of the field, at "Blue," than the red image at "Red."
being nearest to the lens, the green, yellow, red, etc., lying at greater distances in the sequence given. For each color, therefore, an image will be produced in the same way as shown in the previous paragraph having spherical aberration and lying at different distances from the lens. We have to imagine the blue rays forming a cone of the same general shape as just shown, the green ones another one, a little more pointed, because the green image lies farther away from the lens, the red rays forming a still more pointed cone and so on. The precise shape of such a cone is further illustrated in figure 00. A point of a stencil interposed in this multitude of cones will be projected by the red, green, blue and so on rays at different places on the screen because these differently colored rays intersect the optical axis at different places and under different angles. Figure 4 shows how a white ray drawn to pass through the margin of the lens, the blue component, and of another white ray passing nearer to the axis, the red component goes through the transferred point P of the stencil. Figure 5 shows the projection of a transparent point of the stencil. The red ray pro