430 ORGANS OF THE SENSES. 



indefinite distance, and can see clearly those at a distance of 

 25 centimetres. This is the distance, in fact, at which we 

 receive the greatest quantity of light, and the faculty of 

 adaptation usually lies between an indefinite distance and one 

 of 25 centimetres. 



There are, however, sometimes, great differences in this 

 phenomenon ; the limits which we have mentioned are those 



Fig. 116. Hypermetropic eye and myopic eye (far-sighted and near-sighted eye).* 



belonging to eyes in the normal condition, which are called 

 emmetropic. The ocular media of some persons, however, 

 have so little converging power that, whatever be the length 

 of the objective cone, the ocular cone is never sufficiently 

 short to allow its summit to fall upon the retina ; even when 

 the luminous point is at an infinite distance, its image goes 

 beyond the retina ; such persons are called hypermetropio, in 

 other words, the object would have to be placed at distance 

 more than infinite before the summit of its ocular cone could 



* 1, Hypermeiropic Eye. The luminous rays ariving from an infinite dis- 

 tance (parallels) produce an ocular cone, the summit of which falls beyond the 

 retina (at a), either because the cone is too long (lack of converging power in 

 the media of the eye), or because the retina is too far forward (the eye being too 

 short). 



2, Myopic Eye. The luminous rays from an infinite distance (parallels) 

 produce an ocular cone, the summit of which falls in front of the retina fat 6), 

 either because this cone is too short (excess of converging power in the media), 

 or because the retina is placed too far back (the eye being too long). Donders'g 

 researches seem to show that short-sightedness is* owing to this latter cause, as 

 is well shown in the figure : the ocular globe being greatly elongated from back 

 to front). 



