THE KEFRACTION OF THE EYE 535 



the point is reached at which the eye is emmetropic (normal). If however 

 the child is allowed to use its eyes too much for near work, the eyeball 

 goes on increasing in size until it has overshot the mark and has thus caused 

 the eye to become short-sighted (myopic). There would appear to be 

 some kind of automatic control, which causes the eye to grow till it 

 is in adjustment with the conditions most frequently encountered. This 

 hypothesis is confirmed by the fact that if a child which is beginning to 

 develop short- sight is prevented from using near vision for a year or two, 

 the development of short-sight stops. The importance of the early detection 

 of the onset of short- sight therefore cannot be too strongly urged. 



THE METHODS OF DIAGNOSIS. The detection of errors of refraction in the 

 eye may be effected in various ways, eacli of which is said to possess advantage. Some 

 of these have come into such general use that they may be briefly considered as an 

 introduction to the description of the more important types of error which they are used 

 to investigate. 



THE DETERMINATION OF THE VISUAL ACUITY. It has been found by 

 experiment that persons with normal sight can distinguish between objects when the 

 angle separating them is not much less than one minute. Test type has therefore 

 been prepared in which the letters are composed of lines which subtend this angle at 

 the eye, when the typs is placed at a standard distance of six metres. Persons who 

 are able to read the type at this distance are said to have normal vision. Above these, 

 standard letters are placed a series of larger letters, which at two, three or four 

 times the standard distance would subtend the standard angle. A person with reduced 

 acuity might be able to read at six metres the type that should be read at sixty. He 

 therefore has vision which is one-ten% the normal. Such a person might have 

 long-sight, short-sight or astigmatism : to determine which is present a pair of spectacles 

 is placed before his eyes into which can be inserted any two of a large selection of glasses 

 of different power, which are known as trial lenses. These are tried in turn in an orderly 

 manner until some are found which allow the man to read the standard type at the stan - 

 dard distance. His visual acuity is now at the normal and the strength and shape of 

 the glasses in front of his eyes is carefully noted, so that others of the same power may 

 be fitted to spectacles for him to wear. If the glasses are found to be convex (plus), 

 then he was suffering from long-sight (hypermetropia), and if concave (minus) from short- 

 sight (myopia). But if on the other hand cylindrical lenses had to be used, then he 

 had astigmatism, either alone or in conjunction with long- or short-sight. 



THE METHOD OF RETINOSCOPY. Such a method as that just described 

 could only succeed if the person tested were an intelligent adult, because we depend 

 entirely on his giving the correct answer when we ask, if the substitution of a different 

 lens to the one we have already placed before him makes vision better or worse. With 

 a child such a method could never succeed. -Another method is therefore practised, 

 which has the great advantage of being independent of the patient ; in fact for the 

 purpose of the Cest he might be blind. This method consists in throwing into each of 

 his eyes in turn, a beam of light reflected off a plane mirror, in the centre of which is 

 a hole, through which the doctor looks. When the beam of light is directed into the 

 patient's eye the doctor sees a pink reflected beam of light coming to him through 

 the patient's pupil. As the mirror is gently tilted, so as to throw the beam slightly 

 upwards and slightly downwards, so the pink beam appears to move up and down 

 behind the patient's pupil. If it moves down as the mirror is tilted down the move- 

 ment is said to be WITH the mirror, and the patient is hypermetropic, requiring plus 

 spectacles. If on the other hand the beam moves AGAINST, minus spectacles are 

 required since the patient ia short-sighted. By placing glasses of different power in 

 front of his eye until one is found which causes the pink beam to move neither with nor 

 against the mirror, the actual power for the spectacles required by the patient is ascer- 



