A Manual Of Photography

CHAPTER VIL ON LENSES FOK THE PHOTOGRAPHIC CAMERA. It is, to the photographic artist, a matter ot considerable moment that he understands the principles upon which his instruments are constructed. It has, therefore, been thought advisable to add a short chapter which should give a sufficiently popular explanation of the dioptrical phenomena with which we have especially to deal. . Upon the refractive power of the media employed, depends the perfection of the results we obtain ; therefore, some ot the phenomena of refraction, or breaking back, as the term implies, should be clearly understood. A ray of light passing through a vacuum progresses in a perfectly straight line, and we should, if we looked at a brilliantly illuminated point—were it possible—under such conditions, see it in its true position, the numerous rays coming undisturbec directly to the eye. But all matter, however attenuated it may be, has the property of refracting, or bending the ray ot light; consequently we do not see the stars in their true position, owing to the refractive power of the atmosphere. The most simple illustration of refraction is to allow a sun- beam a, passing through a small hole in the window-shutter ot a dark room, to fal1 upon the surface ot a fluid contained in a glass vessel, b 6: instead of proceeding onward to «, it will e found to alter its course at the sur- face of the fluid, and pass along the line to a a. Every substance has different refractive powers in virtue of its physical constitution; but a ray of light incident perpendicularly on a refracting medium, as the ray c, (Fig. 29) suffers no refraction. 11 we float, one upon the other, fluids, b,c,d, having different powers of refraction, we shall then see the relative phenomena exhibited by the