840 CHROMATIC AND SPHERICAL ABERRATION. 



j meter. No. 2 is a lens of twice this strength, namely, 2 D; that is its refractive 

 power = f meter, and its focus = \ meter. No. 3 has three times the strength = 3 D, 

 that is, its refractive power = f meter, and its focus = \ meter. No. 4 is four 

 times as strong = 4 D: its refractive power = f meter, and its focus = \ meter. 

 No. 5 is 5 times as strong = 50, etc. Weaker glasses than i D have been chosen: 

 of 0.75 D, with a focus of 1.33 meter; further, of 0.50 D, with a focus of 2 meters; 

 and 0.25 D, with a focus of 4 meters. Between the whole numbers of diopters \ 

 and \ diopter may, of course, be introduced. 



In cases of recognized myopia or hyperopia, glasses should by all means be 

 worn for the preservation of the eye. If the far point in a case of myopia is 

 beyond 5 inches, the glass may be worn constantly, but generally the distance 

 for ordinary near work, such as reading, writing, and handwork, should always 

 be about 12 inches. If the work is so fine (embroidering, dissection, drawing, 

 etc.) that the object must be held closer to the eye in order to obtain a larger 

 retinal image, the glass may be removed or a weaker one be substituted. The 

 )erope may use his glass for near vision, and especially in a poor light, because the 



diffusion-circles are then unusually large on account of the dilatation of the pupil. 

 It is advisable to choose rather excessively strong convex glasses at first. Cylin- 

 drical glasses will be discussed under Astigmatism. Smoked or blue glasses are 

 worn to protect the eye from unduly intense illumination when the retina is 

 sensitive. Stenopaic glasses consist of narrow diaphragms placed in front of 

 the eye, which compel the eye to look in a definite direction, namely through 

 the opening in the diaphragm. Contact-glasses are discussed on p. 841. 



CHROMATIC AND SPHERICAL ABERRATION. 



DEFECTIVE CENTERING OF THE REFRACTING SURFACES. ASTIG- 

 MATISM. 



Chromatic Aberration in the Eye. All rays of white light that undergo refrac- 

 tion are at the same time decomposed into the prismatic colors of which white 

 light is composed, because these colors possess different degrees of refrangibility. 

 The violet rays are refracted the most, the red rays the least. A white point 

 on a black ground does not form a sharp, simple image on the retina; many 

 colored points are formed instead, one behind the other. If the eye is accom- 

 modated to focus the violet rays, the succeeding colors must yield concentric 

 diffusion-circles, those near the red being the largest. In the center of the circles, 

 where all the colors are superposed, a white point is formed by their union, while 

 around it are the colored circles. The distance of the focus of the red rays from 

 that of the violet rays in the eye is from 0.58 to 6.62 mm. v. Helmholtz calculated 

 the focus for red in the reduced eye as 20.524 mm., that for violet as 20.140 mm. 

 Therefore, both the near and far points for violet light are closer to the eye than 

 those for red. Hence white objects beyond the far point seem to have a reddish 

 tinge; those within the near point a violet shade. The eye must also accom- 

 modate more strongly for red rays than for violet ; so that red objects are thought 

 to be closer at hand than equally distant violet objects. This fact should be 

 taken into account by artists. 



Monochromatic or Spherical Aberration. Apart from the decomposition of 

 white light into its components, the rays from a point of simple light are prevented 

 from coming to a single focus by the fact that the edges of refracting (even though 

 only approximately) spherical surfaces refract the rays much more strongly 

 than do the middle portions. Many images are, therefore, formed, instead of 

 one. In the eye this defect is naturally corrected by the iris, which cuts off the 

 marginal rays (Fig. 289), especially when the lens is strongly curved, and at the 

 same time the pupil is most contracted. The marginal part of the lens, in addition, 

 has less refractive power than the central nucleus. Finally, the marginal portions 

 of the refracting surfaces in the eye are less curved than those lying nearer the 

 optic axis, as will be seen by comparing the form of the cornea (p. 815) and that 

 ot the lens-surfaces (p. 821). 



Defective Centering of the Refracting Surfaces. The absence of exact center- 

 ing ot the refracting surfaces in the eye disturbs somewhat the sharp projection 

 t the image. The vertex of the cornea does not lie exactly at the end of the 

 optic axis. The same is true of the vertices of the lens and also of the various 

 layers ot the lens. The deviations and the visual disturbances produced by them 

 are, it is true, usually but slight. 



