PHYSIOLOGY 



tained. It should be carefully noted however that, since the doctor is standing at 

 about a meter distance from the eyes of his patient, plus one D must be subtracted from 

 the power of any glasses that are found to be necessary. Thus if the patient was 

 found to be myopic and minus 7 D spherical lenses were required to neutralise the 

 movement of the beam, then the power that should be ordered is minus 8 D spherical. 

 This test is found to work admirably in practice. It is better to have paralysed the pupil 

 reflex and the accommodation of the patient previous to the test by the use of atropine, 

 but some say that this is unnecessary. 



OTHER METHODS. Of other methods of testing vision little requires to be said : 

 some require the use of special instruments such as the optometer and the refractometei . 

 Another again depends on the determination of both the far and near points. This is 

 of distinct value because it at once gives the amplitude of the accommodation, which 

 is Jin important determination. Others are based on the use of the ophthalmoscope. 

 But none of these methods are so simple or accurate as the method of retinoscopy 

 described above. 



STENOP^EIC APERTURE. Often in practice the question arises as to whether 

 low visual acuity is due to defect in the optical media of the eye, or to disease of the 

 retina. This question can be readily answered by placing in front of each eye in turn 

 a metal disc in which has been drilled a one-third millimeter hole. If this improves acuity 

 the defect is not in the retina ; if it does not it is. This test should be done in a good 

 light because of the small amount of light passed by the hole. A hole used in this way 

 is called a stenopsoic aperture. 



HYPERMETROPIA OR LONG-SIGHT. There are two principal varieties of 

 long-sight, firstly that in which the eyeball is too small and too short for the normal 

 optical system, secondly that in which the eyeball is normal but the refracting power 

 of the lens below the normal. The first variety is found in childhood, because the optica I 

 system reaches its adult size much earlier than does the eyeball. In the majority of 

 children the eyeball continues to grow until iins the correct size, and therefore long-sight 

 disappears. In a certain number of cases this does not happen and therefore long-sight 

 remains through life. The second variety is found in old age, and appears to be due to 

 the absorption of water by the lens; the result in both cases being that the rays of 

 light from distant objects are brought to a focus behind the retina, and therefore in order 

 lo focus them the accommodation has to be used (see Fig. 267). It follows from this 



Kh;. 2(\1. Hi/I" run //</>/( eye. 

 The eyeball is too short' and the re to 

 from a distant object come ( 



foi ii I c\ i, ml i he retina. 



FIG. 268. Myopic eye. 

 The eyeball is too long and therefore 

 ry fnmi a distant object come to a 

 Incus in It.uit \ the retina. 



