314 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. 129. 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- 



^^^^^~~~ 



Fig. 130. Reflected images of a candle flame as seen in the pupil of an eye at rest and 

 accommodated for near objects. (Williams.) 



fleeted from the observed eye as from a mirror: one, the brightest, 

 is reflected from the convex surface of the cornea (a, Fig. 130, A) ; 

 one much dimmer and of larger size is reflected from the convex 

 surface of the lens (6, Fig. 130, A). This image is larger and 

 fainter because the reflecting surface is less curved. The third 

 image (c, Fig. 130, 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 gazes at a near object, it will 

 be noted (Fig. 130, J5) that the first image does not change at 

 all, the third image also remains practically the same, but the 

 middle image (b) becomes smaller and approaches nearer to the 

 first (a). This result can only mean that in the act of accom- 

 modation the anterior surface of the lens becomes more convex. 

 In this way its refractive power is increased and the more diver- 

 gent rays from the near object are focused on the retina. Helm- 

 holtz 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 

 between the choroid and the sclerotic coat at the point at which the 

 sclerotic passes into the cornea and the choroid falls into the ciliary 

 * Helmholtz, "Handbuch der physiologischen Optik," second edition, 1896. 



