THE PUPIL. 1043 



existing between stimulus and sensation, namely, that the amount of 

 contraction of the pupil is proportional to the logarithm of the intensity. 

 Schirmer x found the condition of adaptation of the eye to be of great 

 importance, and that, in different conditions of adaptation of the two 

 eyes, the same illumination may contract the pupil of one, and dilate 

 that of the other eye. With maximal adaptation to the dark, Schirmer 

 found that the pupil maintained the same diameter, usually between 3 

 and 4 mm., within wide ranges of illumination (i.e. between 100 and 

 1100 mc.). 2 Schirmer has proposed to call this the physiological pupil- 

 width, and Silberkuhl 3 has employed it for measurements of the pupil 

 under different conditions. The pupil is said to contract more when light 

 falls on the central retina than on the periphery. This is probably true 

 under ordinary conditions, but has not been investigated in dark adapta- 

 tion, when a different result would probably be obtained (see p. 1083). The 

 size of the retinal area stimulated has been found to be of influence, but 

 there are no exact observations on this point also. The pupil has been 

 said to become smaller and to contract less with advancing age. It is 

 also said to be larger in myopic than in emmetropic and hypermetropic 

 eyes, but Schadow 4 and Silberkuhl 5 have failed to find any appreciable 

 difference. Silberkuhl found no difference in cases of unequal refraction. 



Each pupil contracts not only when light falls directly upon it, but 

 also when light falls on the pupil of the other eye. These reactions are 

 known as the direct and consensual reactions respectively. 6 It is said 

 that the consensual reaction takes place somewhat later than the direct, 

 but this point seems to be doubtful. 



The contraction of the pupil with accommodation and convergence is 

 an associated movement. Following E. H. Weber, it is often supposed that 

 the contraction is especially associated with convergence, but this is not 

 the case. The pupil contracts with accommodation without any change in 

 convergence, as well as with convergence without accommodation. The 

 latter may be shown by experiments with prismatic glasses, while the 

 former is shown by an experiment due to Hering, 7 in which the pupil is 

 observed by the entoptic method. Each eye looks through a fine hole 

 in a screen, so that a binocularly combined image is seen ; if a second 

 hole is made in each screen, in one above and in the other below the 

 original hole, three pupillary diffusion circles may be observed in a line, 

 and are seen to change in size when the accommodation is changed 

 without alteration of convergence. According to a recent paper, how- 

 ever, by Verwoort, 8 this result is clue to the imperfect exclusion of 

 convergence ; and this author believes that Weber was right in asso- 

 ciating the reaction of the pupil with convergence (see p. 1131, for 

 the relation between accommodation and convergence). 



Mechanism of contraction. — The contraction of the pupil, due to 

 the action of the sphincter muscle, depends, in birds and mammals, 

 upon a reflex starting from the retina, and having its centre in the 

 floor of the aqueduct of Sylvius. In fishes and amphibia, the pupil con- 



1 Arch./. Ophth., 1894, Bd. xl. Abth. 5, S. 8. 



2 1 mc. is the illumination given by one candle at a distance of 1 metre. 



3 Arch./. Ophth., 1896, Bd. xlii. Abth. 3, S. 179. 



4 Ibid., 1882, Bd. xxviii. Abth. 3, S. 183. 5 hoc. cit. 

 6 Steinach {Arch. f. d. ges. Physiol., Bonn, 1890, Bd. xlvii. S. 289) has found that the 



consensual reaction does not occur in animals with total decussation of the optic nerves in 

 the chiasma. 



7 "Lehre v. binoc. Sehen," 1868, S. 134. 



8 Arch. f. Ophth., 1899, Bd. xlix. S. 348. 



