SENSE OF VISION. 745 



610. There are two circumstances, however, which interfere with the per- 

 fection of an image thus formed by a convex lens. The one is, that if the 

 lens constitute a large part of the sphere from which it is taken, the rays 

 which fall near its margin are not brought to a focus at the same point with 

 those which pass through its centre, but at a point nearer the lens. This 

 difference, which must obviously interfere greatly with the distinctness of the 

 image, is termed Spherical Aberration; it may be corrected by the combina- 

 tion of two or more lenses, of which the curvatures are calculated to balance 

 one another, in such a manner that all the rays shall be brought to the same 

 focus; or by diminishing the aperture of the lens by means of a stop or 

 diaphragm, in such a manner that only the central part of it shall be used. 

 The latter of these methods is the only one employed, where the diminution in 

 the amount of light transmitted is not attended with inconvenience. The 

 nearer the object is to the lens (and the greater, therefore, the angle of di- 

 vergence of its rays), the greater will be the spherical aberration, and the 

 more must the aperture of the diaphragm be reduced in order to counteract 

 it. The other circumstance that interferes with the distinctness of the image, 

 is the unequal refrangibility of the differently colored rays which together 

 make up white or colorless light ; the violet being more bent from their course 

 than the blue, the blue more than the yellow, and the yellow more than the 

 red ; the consequence of which will be, that the violet rays are brought to a 

 focus much nearer to the lens than the blue, and the blue nearer than the 

 red. If a screen be held to receive the image in the focus of any of the 

 rays, the others will make themselves apparent as fringes round its margin. 

 This difference is termed Chromatic Aberration. It is corrected in practice, 

 by combining together lenses of different substances, of which the dispersive 

 power (that is, the power of separating the colored rays) differs considerably. 

 This is the case with flint and crown-glass, for instance, the dispersive power 

 of the former being much greater than that of the latter, whilst its refrac- 

 tive power is nearly the same ; so that, if a convex lens of crown-glass be 

 united with a concave of flint whose curvature is much less, the dispersion 

 of the rays effected by the former will be entirely counteracted by the latter, 

 which diminishes in part only its refractive power. 1 



611. The refractive surfaces of the eye are three in number, and may be 

 regarded as spherical, though the cornea has been shown to be slightly ellip- 

 soidal. The radius of the first refracting surface, which is that of the cornea, 

 is about 7.5 mm. ; of the second, which is the anterior surface of the lens, 

 varies from 7.8-9 mm. ; and of the third, which is the posterior surface of 

 the lens, from 5.3 to 6.9 mm. The distances between these are: from the 

 anterior surface of the cornea to the anterior surface of the lens varies from 

 2.9 to 3.1 mm. ; the distance from the anterior surface of the cornea to the 



1 The most important recent work on the optical relations of the eye is Helrnholtz's 

 Handbuch der Physiologischen Optik, Leipzig, 1867. MM. Hiippel and Griinhagen 

 have shown (Archives fur Ophthalmoi., Bd. xiv, p. 219) that the ordinary pressure 

 of the contents of the globe of the eye in the cat is sufficient to support a column of 

 mercury of from 22-24 mm. in height. When the third nerve was irritated the pres-~ 

 sure rose from this average to 70-90 mm., which was apparently due to the contrac- 

 tion of the recti and obliquus superior. Irritation of the branches supplying the 

 sphincter iridis and ciliary muscle exerted no influence. Excitation of the cervical 

 sympathetic was followed by a primary exaltation and a secondary diminution of 

 pressure. The former effect, amounting to 1-6 mm., was probably due to the contrac- 

 tion of the orbital muscle; the latter appeared to result from contraction of the ocu- 

 lar vessels. Monniki (Tonometers en Tonometrie, Inaug. Dissert., Abst. in Syd. 

 Soc. Year- Book, 1871-72, p. 339) ascertained that the tension of the eye varied within 

 rather wide limits. It is normal when a pressure of 150 to 225 grains produces a 

 depression of of a millimetre. 



48 



