174 OPTICAL PRINCIPLES. 



ing through a convex lens do not meet at a single 

 point or focus (PL XII. fig. 20), but form as many 

 foci as there are coloured rays. 



When the spectrum is received upon a convex lens, 

 the coloured rays are brought to a focus, and the light 

 appears again white ; for it is only when the primary- 

 coloured rays are parallel, and seen close together, 

 that they produce the impression of white or colour- 

 less light. The spectra produced by different disper- 

 sive media not only differ in length, but also in the 

 breadth of the coloured spaces not being in the same 

 ratio to each other ; hence the spectra are said to be 

 irrational, or the dispersion is said to be irrational. 



Vision. The visibility of an object depends upon 

 the rays of light which emanate from each point of 

 its surface presented to the eye being brought to a 

 focus upon the ret'ina or expansion of the nerve of 

 sight lining the inside of the back of the eye ; so that, 

 an image of each point being impressed upon the re- 

 tina, the sum of the images forms the compound 

 image of the entire object. 



The manner in which the image is formed is 

 shown in PL XII. fig. 21, in which, to prevent con- 

 fusion, the rays coming from three points of the arrow 

 only have been represented. The rays diverging from 

 these three points, a b c, form cones in contact by 

 their bases ; the apex of each cone outside the eye 

 being situated at the points a b c, the common base 

 of each being situated at the crystalline lens x, im- 

 mediately behind the pupil or rounded aperture in 

 the coloured curtain of the eye, called the iris, i i, and 

 which limits the base of the cones. The apices of the 

 cones within the eye, a'b'c', are formed by the rays 

 brought to foci upon the retina by the crystalline lens. 

 The marginal cones of rays coming from the object 

 cross within the eye, so that the uppermost rays from 

 the object become lowermost upon the retina, and thus 

 an inverted image of the object is formed. This ap- 



