An American text-book of physiology . scope.—Under normal conditions the pupil of the eyeappears as a black spot in the middle of the colored iris. The cause of thisdark appearance of the pupil is to be found in the fact that a source of lightand the retina lie in the conjugate foci of the dioptric apparatus of the any light entering the eye that escapes absorption by the retinal pig-ment and is reflected from the fundus must be refracted back to the sourcefrom which it came. The eye of an observer who looks at the pupil fromanother direction will see no light coming from it, and it
An American text-book of physiology . scope.—Under normal conditions the pupil of the eyeappears as a black spot in the middle of the colored iris. The cause of thisdark appearance of the pupil is to be found in the fact that a source of lightand the retina lie in the conjugate foci of the dioptric apparatus of the any light entering the eye that escapes absorption by the retinal pig-ment and is reflected from the fundus must be refracted back to the sourcefrom which it came. The eye of an observer who looks at the pupil fromanother direction will see no light coming from it, and it will therefore appearto him black. It is therefore evident that the essential condition for perceivinglight coming from the fundus of the eye is that the line of vision of theobserving eye shall be in the line of illumination. This condition is fulfilledby means of instruments known as ophthalmoscopes. The principles involvedin the construction of the most common form of ophthalmoscope are illustrateddiagrammatically in Figure Fig. —Diagram to illustrate the principles of a simple ophthalmoscope (after Foster). The rays from a source of light L, after being brought to a focus at a bythe concave perforated mirror M M, pass on and are rendered parallel by thelens /. Then, entering the observed eye B, they are brought to a focus on theretina at a. Any rays which are reflected back from the part of the retinathus illuminated will follow the course of the entering rays and be brought toa focus at a. The eye of an observer at A, looking through the hole in themirror, will therefore see at a an inverted image of the retina, the observationof which may be facilitated by a convex lens placed immediately in front ofthe observers eye. * Schirmer: Archiv/iir Ophlhalmologie, xi. 5. THE SENSE OF VISION. 773 The fundus of the eye thus observed presents a reddish background onwliich the retinal vessels are distinctly visible. Retina.—ITavin<!; considered the mechanism by wh
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