632 THE SENSES. 



the eyeball and the sphincter pupillae are relaxed, but the changes of 

 accommodation are also interfered with ; and in these instances, accord- 

 ing to Helinholtz, the eye invariably remains adapted for long distances, 

 and cannot be brought to a state of distinct vision for near objects. 

 The evidence in this direction is completed by the well-known facts 

 which accompany the usual diminution or loss of accommodative power 

 with advancing years. In old persons, where this change has taken 

 place, it is the accommodation for near objects which is deficient, while 

 that for distant objects remains perfect. 



III. In accommodation for near cbjects, the crystalline lens becomes 

 more convex, thus increasing its refractive power. This is the essential 

 change upon which all the results of accommodation are directly de- 

 pendent. Its existence was demonstrated by Cramer and Donders, 1 by 

 the aid of what are called the "catoptric 

 images," or images of reflection in the eye. 

 If a brilliant candle flame be so disposed, in a 

 room with dark walls, that its rays fall some- 

 what obliquely upon the cornea of the eye 

 tinder observation, and at an angle of about 

 30 degrees with its line of sight, and if the 

 observer place himself on the opposite side, at 

 an equal angle with the line of sight, three 

 ^fleeted images of the flame will become visi- 

 of reflection, from the surface ble, as in the accompanying figure. 



The first left-hand image (Fig. 199, a) 

 face of the lens. c. inverted which is brightest of all, and upright, is that 

 ie'n.? 08 !ri r 8Ur " reflected from the surface of the cornea. The 



second, 6, which is also upright, but much 

 fainter, is the reflection from the convex anterior surface of the lens; 

 and the third, c, which is tolerably distinct, but inverted, is thrown 

 back from the posterior surface of the lens, acting as a concave mirror. 

 If the person under observation now changes his point of sight, from a 

 distant to a near object, the position of the eyeball remaining fixed, the 

 second image, 6, becomes smaller, and places itself nearer the first. 

 This indicates that the anterior surface of the lens, from which this 

 image is reflected, becomes more bulging, and approaches the cornea: 

 at the same time no change is observable in the other two images, 

 showing that the curvatures, both of the cornea and of the posterior 

 surface of the lens, remain unaltered. 



Helmholtz has made the phenomenon above described much more 

 apparent by employing, instead of a single light, two similar sources of 

 illumination placed in the same vertical line. There are thus produced 

 two catoptric images, one above the other, from each surface of reflec- 

 tion ; and an increase or diminution in convexity of either of these sur- 



1 DONDERS, Accommodation and Kefraction of the Eye, Sydenham edition. 

 London, 1864, p. 10. 



