CH. LVI1I.] SKIASCOPY 831 



a white square on a black ground appears larger than a black square 

 of the same size on a white ground. The phenomenon is naturally 

 more marked when the white object is a little out of focus. 



6. Defective Accommodation Presbyopia. This condition is due to 

 the gradual loss of the power of accommodation which is an early 

 sign of advancing years. In consequence, the person is obliged in 

 reading to hold the book further and further away in order to focus 

 the letters, till at last the letters are held too far for distinct vision. 

 The defect is remedied by weak convex glasses. It is due chiefly to 

 the gradual increase in density of the lens, which is unable to swell 

 out and become convex when near objects are looked at, and also to 

 a weakening of the ciliary muscle, and a general loss of elasticity in 

 the parts concerned in the mechanism. 



The Skiascope or Retinoscope. 



The refractive power of a lens is expressed in terms of its 

 principal focal distance; if this is 1 metre, it is said to have the 

 refractive power of 1 diopter (1 D.) ; a lens 2 D. has a focal length of 

 J a metre, and a lens J D. has a focal length of 2 metres, and so on. 

 The lenses necessary for correcting errors of refraction in an eye are 

 best determined by a simple instrument called a retinoscope ; this is a 

 small circular plane mirror, perforated by a hole in the centre 

 through which the observer looks. If one reflects a spot of light 

 from this on to a flat surface, any movement of the mirror produces 

 a movement of the spot of light in the same direction ; if the surface 

 selected, however, is the eye of another person, the direction of 

 movement of the illuminated spot on the retina may or may not be 

 the same as that in which the mirror is moved, according as whether 

 the observed eye is normal, hypermetropic, or myopic. If the 

 observed eye is just a metre away from the observer, and is 

 emrnetropic, then as the mirror is tilted from side to side the spot 

 moves in the same direction. If a convex lens is placed in a 

 spectacle frame in front of the observed eye, the parallel rays which 

 emerge from the retina are brought to a focus and cross before 

 reaching the eye of the observer. Then the spot will move in the 

 opposite direction to the mirror. A lens of less than 1 D. will not, 

 however, accomplish this reversal ; a lens of more than 1 D. will. 

 So that a lens of 1 D. marks the exact point of reversal. If the 

 observed eye is hypermetropic, the movement of the spot of light is 

 also with the mirror, but stronger lenses than 1 D. must be intro- 

 duced to get the point of reversal. If the lens in any particular 

 case necessary for this purpose is 5 D., then spectacles of 4 D. must 

 be ordered for the patient ; for one always has to subtract 1 D., since 

 that is required to get reversal with the normal eye. 



