Page:Principles of Microscope.djvu/99

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1. Introductory.

We may most conveniently pass from the study of image formation by the vacant aperture, to the study of image formation by means of the lens-armed aperture, by comparing the course and configuration of a beam of light transmitted in a symmetrical manner through a lens-armed aperture with the course and configuration of a beam transmitted through the same vacant aperture.

The course of the beam, meaning thereby in each case the course followed by the axial ray which traverses the centre of the aperture, will in the two cases be essentially the same.

The configuration of the beam will, on the contrary, be very different. In the case of the vacant aperture the beam will, as is shown in Fig. 14, A, be represented upon the screen by aluminous disc which will expand as the distance between the receiving in screen and the aperture increases.

In the case of the lens-armed aperture, the beam may be laid down upon the screen according to circumstances, in the different forms indicated in Fig. 14, B, C, and D, and again with more detail in Figs. 15, 16 and 17.

In Fig. 14, B, and Fig. 15, we have a beam originating at a point lying between the principal focus upon the surface of the lens. The diagram shows that the delimiting rays—and what holds true of these holds true of all the included rays—are bent in by the lens in such a manner as to diminish the angle of divergence of the beam.