DIOPTRIC CHROMATIC ABERRATION. 757 



8 inches requires a concave lens of 8 inches focus, i.e., the concave spectacles No. 8. For the 

 hypermetropic (long-sighted) eye, the focal distance of the strongest convex lens, which enables 

 the hypermetropic eye to see the most distant objects distinctly, is at the same time the distance 

 of the far point from the eye. Example. A hypermetropic eye which can see the most distant 

 objects with the aid of a convex lens of 12 inches focus has a far point of 12 ; the proper 

 spectacles are convex No. 12. 



[Diopter or Dioptric. The focal length of a lens used to be expressed in inches ; and as the 

 unit was taken as 1 inch, necessarily all weaker lenses were expressed in fractions of an inch. 

 In the method advocated by Donders, the standard is a lens of a focal distance of 1 metre 

 (39*370 English inches, about 40 inches), and this unit is called a dioptric. Thus, the standard 

 is a weak lens, so that the stronger lenses are multiples of this. Hence, a lens of 2 dioptrics = 

 one of about 20 inches focus ; 10 dioptrics =4 inches focus ; and so on. The lenses are num- 

 bered from No. 1, i.e., 1 dioptric onwards. It is convenient to use signs instead of the words 

 convex and concave. For convex the sign plus + is used, and for concave the sign minus - . 

 Thus a + 4'0 means a convex lens of 4 dioptrics, and a - 4*0 = a concave lens of 4 dioptrics.] 



In all cases of myopia or hypermetropia, the person ought to wear the proper spectacles. In 

 a myopic eye, when the far point is still more than 5 inches, the patient ought always to wear 

 spectacles ; but generally the working distance, e.g., for reading, writing, and for handicrafts, is 

 about 12 inches from the eye. If the person desires to do finer work (etching, drawing), requir- 

 ing the object to be brought nearer to the eye, so as to obtain a larger image upon the retina, 

 then he should either remove the spectacles altogether or use a weaker pair. 



The hypermetropic person ought to wear his convex spectacles when looking at a near object, 

 and especially when the illumination is feeble, because then, owing to the dilatation of the 

 pupil, the diffusion circles of the eye tend to become very pronounced. It is advisable to wear 

 at first convex spectacles, which are slightly too strong. Cylindrical lenses are referred 

 to under Astigmatism. Spectacles provided with dull-coloured or blue glasses are used to 

 protect the retina when the light is too intense. Stenopaic spectacles are narrow diaphragms 

 placed in the front of the eye, which cause it to move in a definite direction in order to see 

 through the opening of the diaphragm. 



391. CHROMATIC AND SPHERICAL ABERRATION, ASTIGMATISM. 



Chromatic Aberration. All the rays of white light, which undergo refraction, 

 are at the same time broken up by dispersion into a bundle of rays which, when 

 they are received on a screen, form a spectrum. This is due to the fact that the 

 different colours of the spectrum possess different degrees of refrangibility. The 

 violet rays are refracted most strongly ; the red rays least. 



A white point on a black ground does not form a sharp simple image on the retina, but many 

 coloured points appear after each other. If the eye is accommodated so strongly as to focus 

 the violet rays to a sharp image, then all the other colours must form concentric diffusion 

 circles, which become larger towards the red. In the centre of all the circles, where all 

 the colours of the spectrum are superposed, a white point is produced by their mixture, 

 w r hile around it are placed the coloured circles. The distance of the focus of the red rays 

 from that of the violet in the eye = 0*58 to 0'62 mm. The focal distance for red is, accord- 

 ing to v. Helmholtz, for the reduced eye, 20*524 mm. ; for violet, 20*140 mm. Thus, the 

 near and far points for violet light are nearer each other than in the case of red light ; white 

 objects, therefore, appear reddish when beyond the far point, but when nearer than the near 

 point they are violet. Hence, the eye must accommodate more strongly for red rays than for 

 violet, so that we judge red objects to be nearer us than violet objects placed at an equal 

 distance (Brucke). 



Monochromatic, or Spherical Aberration. Apart from the decomposition or dispersion of 

 Avhite light into its components the rays of white light, proceeding from a point if transmitted 

 through refractive spherical surfaces we find that, before the rays are again brought, to a focus, 

 the marginal rays are more strongly refracted than those passing through the central parts of 

 the lens. Hence, there is not one focus but many. In the eye this defect is naturally corrected 

 by the iris, which, acting as a diaphragm, cuts off the marginal rays (fig. 531), especially when 

 the lens is most convex, when the pupil also is most contracted. In addition, the margin of 

 the lens has less refractive power than the central substance ; lastly, the margins of the refractive 

 spherical surfaces of the eye are less curved towards their margins than the parts lying nearer 

 to the optical axis. Compare the form of the cornea (p. 733) and the lens (p. 740). 



Imperfect Centring of the Refractive Surfaces.- -The sharp projection of an image is some- 

 what interfered with by the fact that the refractive surfaces are not exactly centred (Brucke). 

 Thus, the vertex of the cornea is not exactly in the termination of the optic axis ; the vertices 

 of both surfaces of the lens, and even the different layers of the lens itself, are not exactly in 

 the optic axis. The variations, however, and the disturbances produced thereby are very small 

 indeed. 



