536 PHYSIOLOGY 



are brought to a focus at a point behind the retina. Persons so affected can 

 see objects at a distance, but always with some effort of accommodation. 

 If accommodation be paralysed by means of atropine everything will appear 

 blurred. Since accommodation is required even for infinite distance, the 

 greatest possible effort will be insufficient to bring near objects into accurate 

 focus, and the near point of such eyes will be greater than normal. Persons 

 with hypermetropia, or long-sightedness, can therefore see objects at a dis- 

 tance perfectly well, but are unable to read small print, since the point of 

 near vision is too far from the eye to allow the small letters to subtend a 

 sufficiently large angle. 



Both these disorders can be corrected by suitable spectacles. In the 

 case of the myopic eye we need lenses which will convert the parallel rays into 

 divergent rays ; such cases are treated therefore with concave lenses. Con- 

 versely, hypermetropia is treated with convex lenses, which will aid the too 

 feeble refractive power of the eye, and so bring parallel rays to a focus on 

 the retina without any effort of accommodation. 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 typs. 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 recogni- 

 tion of the typa 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 curvature 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 



