Page:Elementary Text-book of Physics (Anthony, 1897).djvu/439

§347] eye, probably from the perception of the effort required to get the object clearly focussed on the retina.

346. Magnifying Power.—To increase the apparent size of an object, and so improve our perception of its details, we must increase the visual angle. This can be done by bringing the object nearer the eye, but it is not always convenient or possible to bring an object near, and even with objects at hand there is a limit to the near approach, due to our inability to see distinctly very near objects. Certain optical instruments serve to increase the visual angle, and so improve our vision. Instruments for examining small objects, and increasing the visual angle beyond that which the object subtends at the nearest point of distinct vision by the unaided eye, are called microscopes. Those used for observing a distant object and enlarging the visual angle under which it is seen at that distance are telescopes. In both cases the ratio of the visual angles, as the object is seen with the instrument, and without it, is the magnifying power.

347. The Magnifying-glass.—Fig. 115 shows how a converging lens may be employed to magnify small objects. The point $$a$$ of an object just inside the principal focus $$F$$ of the lens $$A$$ is the origin of light-waves which, after passing through the lens, are changed to waves having a centre $$a'$$ (§ 337) which, when the lens is properly adjusted, is at the distance of distinct vision. Waves coming from $$b$$ enter the eye as though from $$b'.$$ The object is therefore distinctly seen, but under a visual angle $$a'Ob',$$ while, to be seen distinctly by the unaided eye, it must be at the distance