824 THE SENSE OF SIGHT 



terior portion, leads to a movement of this surface toward the cornea. 

 This change may be studied in any human eye, if the person under 

 observation changes his accommodation repeatedly from far to near 

 objects. The most obvious alteration consists in a forward displace- 

 ment of the margin of the iris, caused by the forward bulging of the 

 anterior surface of the lens. The cornea, on the other hand, undergoes 

 no change whatever. These observations may then be repeated by 

 actually measuring the curvature of these refracting surfaces on far 

 and near vision. 



Changes in accommodation may also be effected by stimulating the 

 excised eye electrically. Most commonly we employ for this purpose 

 the eye of a terrapin, which is adjusted under a 

 magnifying glass by means of two fine needles in- 

 serted vertically through its corneal sclerotic junc- 

 tion. These needles are connected with the 

 secondary coil of an inductorium (Fig. 430). On 

 stimulation with single induction shocks, it will 

 be seen that the iris is pushed far forward into the 

 aqueous humor, while the anterior portion of the 

 lens bulges prominently through the pupillar 

 orifice. 



One of the most interesting proofs of accom- 

 modation has been furnished by Langenbeck. 1 

 D It consists in determining the form, size and posi- 

 tion of the images of a brilliant object reflected 

 FIG. 430. DIAGRAM from the different refracting surfaces of the eye. 

 ILLUSTRATING THE Thus, if a candle is held at a distance of about 50 



CHANGES IN THE LENS ,, . . ,. ,, ,. ,, , 



ON STIMULATION OF THE crn - laterally in front of the eye of the observed 



CILIARY BODY. person, while the observer places himself at an 



A, the eye at rest; angle of 15 to 20 to the visual axis opposite 



fh^t^TorcesTstr the candle, three images of this object will be ob- 



vexity through the tained, namely, 



pupillar orifice, pushing 



the iris forward. (a) A bright upright image from the surface of the 



cornea, 



(6) a large upright but faint image from the anterior surface of the lens, and 

 (c) a small inverted and faint image from the posterior surface of the lens. 



The first is very prominent, while the other two are less distinct, but 

 can usually be seen without much trouble by properly adjusting the 

 position of the candle. With this arrangement, the large, faint up- 

 right image from the anterior surface of the lens, occupies the center 

 of the pupil, while the faint inverted image from the posterior sur- 

 face of the lens lies very close to the margin of the pupil opposite the 

 observer. The relative size and position of these images having been 

 clearly ascertained, the observed person is asked to accommodate 

 'alternately for near and far objects (Fig. 431). When this is done, it 



1 Klin. Beitrage zur Chir. und Ophthalm., Gottingen, 1849; also see: Helm- 

 holtz, Monatsber., Berliner Akad., 1853. 



