290 THE SPECIAL SENSES. 



Helmholtz with the aid of what are known as the images of Pur- 

 kinje. The principle of this experiment is represented by the dia- 

 gram given in Fig. 122. The eye to be observed is relaxed; 

 that is, gazes into the distance. A lighted candle is held to one 

 side as represented, and the observer places his eye so as to 

 catch the light of the candle when reflected from the observed eye. 

 With a little practice and under the right conditions of illumina- 

 tion the observer will be able to see three images of the candle re- 

 flected from the observed eye : one, the brightest, is reflected from 

 the convex surface of the cornea (a, Fig. 123, A) ; one much dimmer 

 and of larger size is reflected from the convex surface of the lens 

 (6, Fig. 123, A). This image is larger and fainter because the re- 

 flecting surface is less curved. The third image (c, Fig. 123, A) 

 is inverted and is smaller and brighter than the second. This 

 image is reflected from the posterior surface of the lens, which acts, 

 in this instance, like a concave mirror. If now the observed eye 



A B 



Fig. 123. Reflected images of a candle flame as seen in the pupil of an eye at rest and 

 accommodated for near objects. {Williams.) 



gazes at a near object it will be noted (Fig. 123, B) that the first 

 image does not change at all, the third image also remains practi- 

 cally the same, but the middle image (6) becomes smaller and ap- 

 proaches nearer to the first (a). This result can only mean that in 

 the act of accommodation the anterior surface of the lens becomes 

 more convex. In this way its refractive power is increased and the 

 more divergent rays from the near object are focused on the retina. 

 Helmholtz has shown that the curvature of the posterior surface 

 of the lens is also increased slightly ; but the change is so slight that 

 the increased refractive power is referred chiefly to the change in 

 the anterior surface. The means by 'which the change is effected 

 was first explained satisfactorily by Helmholtz.* He attributed 

 it to the contraction of the ciliary muscle. This small muscle, 

 composed of plain muscle fibers, is found within the eyeball, lying 



* Helmholtz, "Handbuch der phvsiologischen Optik," second edition, 

 1896. 



