A CCOMMODA TION. 1033 



namely, for irregularities of cornea or lens, — irregular astigmatism; 

 and for differences in the refraction of different meridians of the corneal 

 and lens surfaces, — regular astigmatism. One kind of irregular astigmat- 

 ism is due to the structure of the lens, and shows itself in the " star- 

 like " appearance of any minute luminous point. In most eyes, again, a 

 fine luminous line appears, when carefully examined, to be doubled ; or 

 multiplied still more. This may be observed well in the case of the horns 

 of the new moon. This appearance is supposed to be the basis of some 

 of the recorded cases of monocular diplopia or polyopia. 



Eegular astigmatism may depend on a defect either of the lens or of 

 the cornea, the latter being the more common. The existence and 

 amount of corneal astigmatism may be determined by a modification of 

 the ophthalmometer devised by Javal and Schiotz. 1 Thomas Young 2 

 discovered that the defect in his own case was due to the condition of 

 the lens, by finding that it persisted when the refractive power of 

 the cornea was abolished by covering it with a layer of water. The 

 meridians of greatest and least refraction are always at right angles to 

 one another, but may be at any angle to a vertical section through the 

 eyeball. The defect prevents the occurrence of a true punctiform focus, 

 and produces two linear foci, and between them an interfocal region, in 

 one part of which the focus is approximately circular. This circular 

 focus lies nearer the anterior than the posterior linear focus. In the 

 average emmetropic eye the vertical meridian is more convex than the 

 horizontal, the difference not amounting to more than -5 D, and in most 

 cases of marked astigmatism the vertical is also the more convex meridian. 



Chromatic aberration. — The difference of focus for the ends of 

 the spectrum was calculated by Helmholtz. 3 Using Listing's reduced eye, 

 and assuming that its refraction is equal to that of water, he found a 

 difference of "434 mm., so that an eye accommodated for red at infinite 

 distance would be accommodated for violet at a distance of 71*3 cm. 

 Matthiessen 4 found the distance between the two foci still greater (-58 

 to "62 mm.) ; and Einthoven, 5 who has recently calculated the cardinal 

 points of the eye for different parts of the spectrum, also finds the 

 values with the reduced eye too small. Einthoven found that there was 

 no appreciable difference in the position of the posterior nodal point for 

 different colours. 



The effect of this difference of focus may be most readily observed 

 on looking, with inexact accommodation, through violet glass trans- 

 mitting red rays, especially with a half -covered pupil. It is also seen 

 with fine black or white lines. It is much increased when the pupil 

 is dilated. It is said that chromatic aberration is more easily observed 

 by emmetropic and hypermetropic than by myopic eyes. 



Accommodation. 



Changes in lens.— The changes which have been observed during 

 accommodation will first be described. The anterior surface of the 

 lens increases in curvature, while the curvature of the posterior surface 



1 "Beitr. z. Psychol, u. Physiol, d. Sinnesorg." (Helmholtz Festgruss), Hamburg and 

 Leipzig, 1891, S. 39. 



"Phil. Trans., London, 1801, p. 57. 



3 "Handbuch d. physiol. Optik," 2te Aufl., S. 157. 



4 Compt. rend. Acad. d. sc, Paris, 1847, tome xxiv. p. 875. 



5 Arch. f. d. ges. Physiol., Bonn, 1896, Bd. lxii. S. 166. 



