1042 VISION. 



described by Langer, in which the fibres run meridionally, and on contrac- 

 tion pull the lens backwards. Beer found that the increased pressure on 

 the chamber behind the lens, due to the contraction of this muscle, produced 

 bulging of the lax posterior pole of the eyeball, but that the recession of the 

 retina so caused was too small to neutralise the recession of the lens. Both 

 in fishes and cephalopods the iris was found to have no influence on the 

 process of accommodation. There is no evidence of anything like a mechanism 

 of accommodation in the compound form of eye. 



THE PUPIL. 



Pupillometry. In measuring the size of the pupil, it must be 

 remembered that it is magnified by tbe refractive action of the cornea 

 and aqueous humour. The methods ordinarily used have the disad- 

 vantage that they involve the near approach of an object to the eye, 

 and consequent shading. This difficulty may be overcome for exact 

 measurements by the use of a telescope with a scale (Schadow 1 ), or by 

 observing the image of the pupil reflected from a mirror. Schirmer 2 

 used the image reflected from one side of a prism seen against a scale, 

 which was reflected internally from another side of the prism which had 

 been covered with mercury. Hess 3 employs a method in which the 

 virtual image of a scale appears in the plane of the pupil. The most exact 

 method is with the ophthalmometer, on the same principle as described 

 on p. 1030 (Fig. 374). Two other special methods are useful under certain 

 circumstances. One first employed by Bellarminoff * is to photograph 

 the pupil under different conditions, and for this the magnesium flash- 

 light has been used by du Bois-Eeymond 5 and Haycraft. The other 

 method first employed by A. Tick 6 is to observe the entoptic image of 

 the pupil. A fine hole in a screen at a short distance from the eye will 

 be seen as a diffusion circle, which varies in size with the size of the 

 pupil. If two such holes are used, at such a distance from one another 

 that the two diffusion circles just touch, the diameter of the pupil is 

 approximately equal to the distance between the two holes. The exact 

 relation is influenced by the distance of the holes from the eye, and to a 

 considerable extent by the condition of refraction of the eye. The method 

 is especially useful as a means of detecting slight changes in the size of 

 the pupil; when the circles just touch, enlargement of the pupil will make 

 them overlap, and contraction will make them separate from one another. 



Changes in the pupil. Change in the size of the pupil takes place 

 under four chief conditions, namely (1) with change in the illumination: 

 (2) with change in convergence and accommodation ; (3) in response to 

 sensory stimuli in different parts of the body ; and (4) in connection 

 with psychical changes, and especially emotional conditions. The pupil 

 is also affected by various drugs and in various abnormal conditions of 

 the nervous system. 



The amount of contraction depends on the intensity of the light 

 falling on the eye, and Haycraft 7 has recently shown, using the photo- 

 graphic flash light method, that the relation is of the same kind as that 



1 Arch.f. Ophth., 1882, Bd. xxviii. Abth. 3, S. 183. 



2 Ibid., 1894, Bd. xl. Abth. 5, S. 8. 



3 Ber. u. d. Versamml. d. ophth. Gesellsch., 1893, S. 235. 



4 Arch. f. d. ges. Physiol., Bonn, 1885, Bd. xxxvii. S. 107. 



5 Arch.f. Physiol., Leipzig, 1888, S. 394. 



6 Arch.f. Ophth., 1856, Bd. ii. Abth. 2, S. 70. 



7 In a communication to the Physiological Society not yet published. 



