534 PHYSIOLOGY 



cones is supplied with pigment ; the object of these is clearly to absorb scattered light. 

 In spite of this however we find considerable amounts of light being reflected back 

 again by the retina ; in fact it is this light that enables us to see the retinal nerves 

 and vessels through the ophthalmoscope (see page 554). Tne spherical shape of the 

 eyeball will cause the greater part of this reflected light to travel towards the front of 

 the eye, and to fall on an insensitive layer of iris or retina anterior to the ora serrate. 

 From here it will be reflected again on to the retina, but with such reduced intensity 

 as not to cause stimulation. Light reaching the anterior part of the retina through 

 the pupil would after reflection tend to travel towards the posterior part of the retina, 

 that is the part most sensitive to light. The intensity of these peripheral rays is how- 

 ever diminished in a number of ways: firstly by the eyebrows, cheeks and no.se : 

 secondly by the eye-lashes when the lids are approximated, as they are when looking 

 towards a bright light ; thirdly by the relative smallness of the pupil for oblique rays 

 (the pupil being a slit-shaped instead of a circular opening for such rays). The effect 

 of scattered light in the eye is therefore eliminated in this way. It should be noted 

 that in certain animals, which have very acute night vision, the pigment cells of the 

 choroid at the posterior pole of the eye are iridescent, and form a highly reflecting 

 surface behind the retina, which is called the tapetum. The object of this would seem 

 to be to increase the stimulus of a given intensity of light, but the presence of this 

 reflecting layer must increase the amount of scattered light in the oye, and would 

 therefore appear to be of disadvantage in day vision. 



Halation. This is caused in the camera by the image that has formed on the plate 

 l>ring reflected back again on to the plate from the internal surface of the glass. This 

 is not apparent m the photographic film, because owing to the thinness of the gelatin 

 film the reflected image falls back on to the same part of the plate again. In the case 

 of the retina the reflecting layer must be exceedingly close to the sensitive layer, if 

 indeed the two are not identical ; halation in the eye would therefore appear to l>e 

 negligible in amount. 



Flare. The amount of light reflected by the surface bounding two optical media 

 increases as the difference between their refractive indices increases, and also with 

 the angle which the light makes with the surface. Therefore, other things being equal, 

 the smaller the angle of incidence and the more nearly the refractive indices are ident i- 

 oal, the less the amount of flare will be. In the eye the lens system is as it were immersed 

 in a medium of almost the same refractive index as itself : and further even that dill' 1 ! 

 ence is reduced by the fact that the actual refractive index of the crystalline lens is 

 very much less than its equivalent R.I. owing to its peculiar structure. Flare in tin- 

 eye must therefore be quite inconsiderable in amount. 



Irradiation. That this phenomenon is present in the eye may be shown direetly 

 l\ experiment, If for example an electric light filament be looked at before and after 

 the switching on of the current, the increase in thickness on illumination is obvious. 

 . Its cause appears to be imperfect definition, spreading of the light from one retinal 

 element to its neighbours, or spreading of the nerve impulse either at the retina or 

 even in tin- brain itself. 



ABNORMAL REFRACTION OF THE EYE 



It would almost be anticipated that such a complicated organ as 

 the eye would be found to show individual abnormalities. A further con- 

 sideration would probably suggest to us that, considering the smallness of the 

 change that is necessary in any one of the optical media in order completely to 

 destroy definition, it is nothing short of astonisldng that abnormality of refrac- 

 tion is relatively so uncommon. In the newlv born the eye is almost al\\ 

 long-sighted (hypermetropic) ; this is due to the eyeball being too small for 

 the optical system which it contains : t he image formed by the latter is there- 

 fore focussed behind the retina. As age advances the eyeball grows until 



