772 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



amount of light falling upon the retina is not the only factor in determining 

 the size of the pupil. In fact, if the light acts for a sufficient length of time 

 the pupil may have the same size under the influence of widely different 

 degrees of illumination. 1 



This so-called " adaptation " of the eye to various amounts of light seems 

 to be connected with the movements of the retinal pigment-granules and with 

 the chemical changes of the visual purple, to be more fully described in con- 

 nection with the physiology of the retina. 



The Ophthalmoscope. Under normal conditions the pupil of the eye 

 appears as a black spot in the middle of the colored iris. The cause of this 

 dark appearance of the pupil is to be found in the fact that a source of light 

 and the retina lie in the conjugate foci of the dioptric apparatus of the eye. 

 Hence 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 source 

 from which it came. The eye of an observer who looks at the pupil from 

 another direction will see no light coming from it, and it will therefore appear 

 to him black. It is therefore evident that the essential condition for perceiving 

 light coming from the fundus of the eye is that the line of vision of the 

 observing eye shall be in the line of illumination. This condition is fulfilled 

 by means of instruments known as ophthalmoscopes. The principles involved 

 in the construction of the most common form of ophthalmoscope are illustrated 

 diagrammatically in Figure 231. 



M 

 FIG. 231. 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 by 

 the concave perforated mirror M M, pass on and are rendered parallel by the 

 lens I. Then, entering the observed eye B, they are brought to a focus on the 

 retina at a'. Any rays which are reflected back from the part of the retina 

 thus illuminated will follow the course of the entering rays and be brought to 

 a focus at a. The eye of an observer at A, looking through the hole in the 

 mirror, will therefore see at a an inverted image of the retina, the observation 

 of which may be facilitated by a convex lens placed immediately in front of 

 the observer's eye. 



1 Schirmer : Archivfur Ophlhalmologie, xi. 5. 



