THE CILIARY BODY AND LENS 



827 



constrictors of the pupil and retain the eye in a condition adapted for 

 far vision. Physostigmin, on the other hand, stimulates this muscle 

 and renders the eye near-sighted. V 



Scheiner's Experiment.— An experiment which illustrates the 

 process of accommodation, as well as the projection of visual impres- 

 sions, is the one described by Pater Scheiner. Two small holes are 

 made in a cardboard, the distance between them being less than the 

 diameter of the pupil. The eye then looks at two pins placed one 

 behind the other, at a distance of 18 cm. and 60 cm. respectively. 



Fig. 435. — Diagram to Illustrate Schehster's Experiment. 

 The continuous lines indicate the course of the rays from the object for which the 

 eye is accommodated. 



If one pin is focalized sharply, the other appears double. A glance 

 at Fig. 435 will show why this must be so. Thus if the eye is directed 

 upon the near pin A , the far pin B is brought to a focus in the vitreous 

 humor after which the rays again diverge and strike the retina in two 

 places, C and D. If opening 1 is now blocked, the lower retinal image D 

 disappears and hence, also the image projected into space on the side 

 of the block. If the eye is now accommodated for the far pin B, the 

 near pin A is focalized behind the retina, the still divergent rays 

 striking the retina in points E and F. It will be seen that blocking 

 opening 1 will now cut out the upper retinal image E, and hence, the 

 image projected into the visual field opposite to the block. 



The Changes in the Shape and Refractive Power of the Lens. — 

 The changes in the shape of the lens may be deduced from the following 



^ One of the earliest theories pertaining to accommodation proposes that this 

 process necessitates a lengthening and shortening of the entire eyeball, brought 

 about- by the pressure of the contracting extrinsic muscles of the eye. By a 

 process of exclusion, we have shown above that the lens is the essential factor 

 concerned in accommodation. It would be difficult to disprove the facts brought 

 forth in support of this view ; moreover, since an accommodation for an object held 

 at a distance of 15 m., would entail a lengthening of the eyeball of not less than 2 

 mm., it seems hardly possible that such a change could be brought about by means 

 of the normally contracting muscles of the eyeball. Electrical stimulation, per- 

 formed under experimental conditions, might, however, accomplish this end. 

 This view has recently been resurrected by Bates (New York Med. Jour., ci, 1915). 



