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212 HANDBOOK OF PROJECTION FOR
direction must be along a line which will pass through the point in the object and a point in the object plane (see Fig. 61) the same distance from the optical axis that the refracted line passed through the image plane, although the ray may actually have its origin on either side of the object plane and at any practical distance from the object plane.
It is thus seen that if we desire to trace the path of any particular ray in a diagrammatic way, as illustrated in Fig. 62, we would first ascertain the E. F. of the combination, or the focal length if it be a single lens, and would establish lines A-B, Fig. 62, distant from the lens plane by twice the E. F. or focal length of the lens combination. We would then draw a line through a point of the source to some point of the lens plane and would continue the line to the object plane. If we then draw broken line D, Fig. 62, joining points E and F, and continue it through the image plane line, we have but to draw a line from the point where line G joins the lens plane, to and through the point where broken line D joins the image plane, to have the direction of the refracted ray, which will continue in that exact direction until it is intercepted.
Fig. 61 shows how to lay out the diagram, both object and image planes being twice the E. F. of the lens from the lens plane, and parallel thereto. Fig. 62 shows how a single ray is refracted.
Fig 63 shows how a bundle of rays are refracted, and Fig. 64 illustrates the method of finding the image of any point of the object.
First draw line 1, Fig. 64, through a point in the object, and continue it until it joins the object and lens planes. Having thus established the point in the object plane, we next draw a line from it to the image plane, so that it passes through the optical axis line at the lens plane. This is line No. 2 in Fig. 64, and gives us the point of the image plane through which the refracted ray, 1-R, Fig. 64, will pass, whereupon a line, No. 3, Fig. 64, drawn through the object point and the lens plane at the optic axis, and on to the point where it meets refracted ray 1-R will indicate the image point. The terms Object and Image planes are used merely to identify the planes as shown in Fig. 61. And it should be distinctly understood that the object and image plane herein referred to have nothing to do with the position of the actual object and image.
It will apply to either single lenses or simple combinations.