THE SENSES 557 



passing through the circumference of the lens coming to a focus 

 earlier than those passing near the centre. This defect, known as 

 1 spherical aberration,' is remedied in the eye by the introduction 

 of a diaphragm or iris, which prevents some of the rays of light from 

 passing through the circumference of the lens ; spherical aberration 

 is further prevented by the fact that the refractive index of the central 

 part of the lens is greater than that of the circumference. Spherical 

 aberration produces indistinctness of vision by the production of 

 circles of diffusion, caused by those rays which meet too early crossing 

 each other and forming a circle. 



Chromatic Aberration is due to the decomposition of white light 

 into its primary colours by passing through a prism or a convex 

 lens — viz., a spectrum is formed. The colours of the spectrum are 

 differently refracted, the red being the least bent, the violet the 

 most ; when, therefore, the red is distinctly seen, the eye is not 

 focussed for the violet. There is no compensation in the eye for 

 chromatic aberration. The defect is usually not noticeable, because 

 it is small in amount, and is rapidly corrected by alterations in 

 accommodation . 



Schematic and Reduced Eye. — When a ray of light enters the eye, 

 it has to pass through four surfaces, and, including the air, four media. 

 There are two surfaces to the cornea, anterior and posterior, and 

 two surfaces to the lens, anterior and posterior ; each of these 

 surfaces differs in curvature. As media, there are the aqueous and 

 vitreous humours and the crystalline lens ; the latter is further 

 complicated by not being of the same refractive index throughout. 

 The formation of an image in such a complex optical system would 

 be difficult to understand, were it not possible to construct theoretic- 

 ally from it a simplified eye, or, as it is known, a schematic eye. 

 The basis of its construction is, that so long as a complex system 

 has its surfaces and media ' centred ' — that is, symmetrically disposed 

 around the optical axis — it is possible to deal with it as if it consisted 

 of two surfaces and two media — viz., the schematic eye — and even 

 to simplify it still further to one surface and two media, the reduced 

 eye, the media in the latter being air and water. In such a simple 

 optical system it is readily possible to trace the paths taken by the 

 rays of light, and so understand the formation of an image on the 

 retina of the eye. 



Cardinal Points. — The most simple optical system which can be 

 devised has an optic axis (OA, Fig. 176) — viz., a line passing through 

 its centre perpendicular to its refractive surface (apb). On the optic 

 axis is situated the centre of curvature of the refracting surface ; this 

 centre is known as the nodal point n. All rays of light which strike 

 the refractive surface perpendicularly, such as O, m, pass through 

 the nodal point and are not refracted ; all rays of light parallel to 

 the optic axis, such as cd, strike the refractive surface obliquely 

 and are refracted, and the point where they meet is called the 

 principal posterior focus, F 2 . On the optic axis, in front of the re- 

 fractive surface, is situated a point Fj^ known as the principal 

 anterior focus ; rays proceeding from this point strike the surface 

 obliquely, and are so refracted as to be rendered parallel (ef) to the 

 optic axis (OA). To these must be added the principal point p, 

 that is, the point where the refracting surface cuts the optic axis. 

 These various points are known as the cardinal points of the simple 

 optical system we have imagined. For a more complex system 

 such as the eye, even when simplified, there are two nodal points, 



