1038 VISION 



it occurs in cases of unequal refraction. He combined printed type in the 

 stereoscope, arranging that the gaps in the type presented to one eye should 

 be supplied by that presented to the other eye, and found that the print could 

 be read with a difference of 2 to 3 D. Hess l has, however, by means of 

 careful stereoscopic experiments, shown that the older view is probably 

 correct. He used fine threads at slightly different distances, so that they would 

 be seen lying side by side in binocular vision, and found a difference of distance 

 amounting to '25 D was sufficient to render one of the two threads indistinct 

 or even invisible. He found that cases of slight anisometropia showed no 

 more evidence of unequal accommodation than did normal vision ; when fully 

 corrected, a difference of *5 D was not compensated for by accommodation. 

 Hess ascribes Fick's results partly to the use of too coarse tests, and partly 

 to his neglect of the influence of the pupil. It is possible in myopia for a 

 distant object to appear more distinct with the eye accommodated for near 

 vision than with the unaccommodated eye, owing to the contracted pupil 

 in the former case more than compensating for the increased diffusion circle. 



Hess also calls attention to the influence of rivalry in such experiments 

 (seep. 1122). 



Greeff 2 investigated the same question by a different and very simple 

 method. If an opaque body is held before the eyes, part of the visual field 

 will be seen by one eye, part by the other eye, and part by both eyes. With 

 different refraction in the two eyes, one part of the field will be indistinct if 

 the difference is not compensated for by accommodation ; and Greeff found by 

 this method that both in natural and artificial anisometropia there was not 

 the slightest evidence of unequal accommodation. 



The evidence seems also to be against the existence of meridional accom- 

 modation. Its occurrence has been advocated (1) on the ground of experiments 

 in which astigmatism is imitated by means of glasses, (2) on account of the 

 marked and often sudden changes of vision which occur in cases of natural 

 astigmatism, and (3) because astigmatism is sometimes found to have increased 

 after instillation of atropin. Hess 3 has recently failed to find any evidence of 

 unequal contraction of the ciliary muscle sufficient to overcome astigmatism of 

 a fraction of a dioptre. Astigmatics often assist vision by closing the eyelids, 

 so as to convert the palpebral aperture into a slit, and this fact may explain 

 some of the cases which have been brought forward in favour of meridional 

 accommodation. Hess has called attention to another possibility. It has 

 generally been supposed that astigmatics always accommodate for one or other 

 of their linear foci. Hess finds that this is not the case, but that accommoda- 

 tion normally occurs for that point in the interfocal region where the focus 

 is approximately circular. Using a photographic apparatus rendered astigmatic 

 by a cylindrical lens, Hess 4 found that the clearest image occurred at the 

 circular focus ; and careful observations of the eyes of astigmatics showed that, 

 when they saw most distinctly, their refraction was adjusted approximately to 

 the circular focus. The variations in astigmatic vision, which have been 

 ascribed to meridional accommodation, may very well have been due to 

 variations in the adjustment of the eye to different parts of the interfocal 

 region. If meridional accommodation occur in cases of astigmatism, it might 

 be expected to lead to anatomical changes in the form of localised hypertro- 

 phies of the ciliary muscle. No such appearances have been recorded. 



Theories of accommodation. In the widely accepted theory of 

 Helmholtz, it is supposed that during relaxation of accommodation 

 the suspensory ligament is in a state of tension, thereby compress- 



1 Arch.f. Ophth., 1889, Bd. xxxv. Abth. 1, S. 157. 



z Arch.f. Augenh., Wiesbaden, 1891, Bd. xxiii. S. 371. 



3 Arch.f. Opktti., 1896, Bd. xlii. Abth. 2, S. 80. 



4 Loc. cit. Refer to Hess's paper for the photographs illustrating this. 



