328 PHYSIOLOGY CHAP. 



view of the fundus, if the distance between the correction lens 

 and the observed eye is constant, the eye of the observer being 

 always emmetropic and its accommodation relaxed. 



But as a matter of fact perfect emmetropia is rare, and in near 

 vision we are apt to make more or less use of our ciliary muscles, 

 so that in determining static refraction by the ophthalmoscope 

 it is also necessary to know the refraction of the observing eye. 

 If x is the refraction of the correction lens, y the refraction of 

 the observer's eye, r the static refraction of the observed eye, then 

 T = X- y. 



Ophthalmoscopy is not, however, the most practical, rapid, or 

 certain method for the exact estimation of the static refraction 

 of an eye. Skiascopy, invented by the French oculist Cuignet 

 (1873-74), and developed by Leroy (1884) and Parent (1880-87), 

 gives far more accurate results. 



We have seen that when a pencil of light is thrown by an 



FIG. 153. Displacement of the image of a flame reflected from a plane mirror into the fundus of 

 the eye, by gently rotating the mirror round an axis passing through its central aperture. 



ophthalmoscopic mirror into the pupil, the latter appears red. 

 If the observer then rotates the mirror a shadow is seen to pass 

 across the illuminated pupil from left to right, or from right to 

 left, from above downwards or from below upwards, according to 

 the direction of the movements given to the mirror. By the 

 observation of these shadows the static refraction of the observed 

 eye can be quickly determined. 



Skiascopy is usually practised with a plane mirror. The 

 observer places himself 1 metre away from the observed eye, 

 into which he throws the beam of a lamp placed at the level of 

 one of the subject's ears. On rotating the mirror round an axis 

 passing through its centre, corresponding movements of the light 

 rays and shadows in the pupil are seen. Fig. 153 shows that 

 when the mirror moves from mm to m'm', the virtual image of the 

 flame is displaced from/ to/', and the retinal image from i to i'. 



The rays of light penetrate the observed eye as if they came 

 from the reflected image, which as the mirror has a plane 



