596 PHYSIOLOGY 



modation. The degree of myopia or hypermetropia is denoted by 

 the refractive power of the lens which is necessary to make the eye 

 emmetropic. 



In order to determine the refractive power of any eye it is usual to employ 

 Snellen's test type. This consists of a series of letters which are placed at a 

 distance of five metres from the eye. At this distance the visual angle subtended 

 by each of these letters is so small that a clear retinal image is necessary for 

 their recognition. This is easy in the case of a normal eye. After allowing 

 the patient to attempt the recognition of the type without spectacles he is 

 then made to regard it through a weak convex lens. If the patient can now 

 read as well as or better than before he is hypermetropic, since it is only the 

 hypermetropic eye which is able to unite convergent rays of light on to the retina. 

 If, on the other hand, the reading of the type is made more difficult the patient 

 is either normal (emmetropic) or myopic. In the latter case a concave lens is 

 tried. If the reading is rendered more easy by this means the patient is myopic. 



In prescribing the lenses for hypermetropia, the strongest lens with which 

 the patient is able to see represents the degree of hypermetropia. Since now 

 the mechanism for accommodation must be relaxed as far as is possible, the 

 strength of such a lens serves as a measure of the degree of hypermetropia. 

 On the other hand, in myopia the degree of the disorder is determined by the 

 weakest lens, by means of which the patient is able to see distant objects. 



In a perfect dioptric mechanism the media through which the 

 light passes must be perfectly transparent, and the centres of curva- 

 ture of the various refracting surfaces must lie in one straight line, 

 i.e. the system must be properly centred. In neither of these respects 

 can the eye be regarded as perfect. If a strong beam of light be 

 thrown into the eye, the refraction of the beam caused by the slight 

 difference in structure between adjacent portions of the cornea and 

 lens makes these objects immediately visible, and the field of vision 

 is filled with diffused light arising from the illuminated points in these 

 structures. Under normal circumstances, however, these slight 

 differences in the regularity of the refracting media do not make any 

 appreciable difference to our vision. More easily detected are the opacities 

 due to structures in the vitreous humour. These opacities can be 

 seen, when the eyes are turned towards a uniformly illuminated surface, 

 as small dark points or strings of beads, which, since they alter their 

 position with changes in the direction of the eyes, are often spoken of as 

 musccs volitantes. The centring of the eye is also never perfect. In the 

 horizontal meridian the optic axes of the cornea only diverge about 

 O3 from the axis of the lens, but in the vertical meridian there is 

 as much as 1-3 difference between the two axes. Moreover the visual 

 axis does not correspond exactly with the optic axis of the eye. The 

 fovea centralis, the point of distinct vision on which the image of any 

 object must be brought in order to see it as distinctly as possible, always 

 lies outside and somewhat below the point at which the optic axis 

 strikes the retina. The angle between the two axes is often spoken 



