The elements of physiological physics The elements of physiological physics: an outline of the elementary facts, principles, and methods of physics; and their applications in physiology elementsofphysio00mgre Year: 1884 Pff 1 -'SIOL OGICA L PH I 'S/CS. [Chap. XXIX. are pierced close to one another. The card is held close to the eye, and in front of it is held a needle. On moving the needle nearer to the card and then farther from it, a position is found where it is distinctly seen. If it be brought slightly nearer, the needle appears double, and the same thing happens if it be moved away a li
The elements of physiological physics The elements of physiological physics: an outline of the elementary facts, principles, and methods of physics; and their applications in physiology elementsofphysio00mgre Year: 1884 Pff 1 -'SIOL OGICA L PH I 'S/CS. [Chap. XXIX. are pierced close to one another. The card is held close to the eye, and in front of it is held a needle. On moving the needle nearer to the card and then farther from it, a position is found where it is distinctly seen. If it be brought slightly nearer, the needle appears double, and the same thing happens if it be moved away a little from its first position. The explanation is evident from Fig. 172, where A and B represent the holes in the card, a the point of the needle; c represents a lens, and D, E, and F, a screen at Pig. 172.—Schemer's Experiment varying distances from it. With the screen at E, a distinct single image of the needle is perceived, because the rays from A and B coincide, and are focussed at o ; at the position F, the image is blurred and double, because the rays from A do not coincide with those from B, while at D the image is also double and blurred, because the rays are intercepted after they have diverged from their focus. With the screen in a fixed position, the same effects are pro- duced by varying the distance of a from the screen. Let C and the screen represent the refractive media of the eye and the retina, the explanation applies, and the phenomena of diffusion images are understood. It is evident, then, that the eye in its condition for focussing parallel rays will produce on the retina images of diffusion with divergent rays, because the focal point is thrown behind the retina. It is equally evident that if an increase of refractive power were given to the media, the focal point would be brought forward and made to coincide with the retina. Every different distance of the object looked at would require a new adjustment. The increased refractive power would be conferred
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