xc INTRODUCTION TO OPTICS. 



rays diverge originally, they will be less convergent or more divergent 

 after refraction, but they would in general still finally meet at a point, or 

 appear to diverge from one. Taking the case of a convex lens, the 

 point at which they would meet would be farther from the lens than that 

 in which parallel rays meet, and continually farther and farther, as the 

 rays were more divergent, or as the body from which they proceeded was 

 brought nearer to the lens. An image* would therefore be formed, but 

 continually farther and farther from the lens, as the body approached it ; 

 and the image is smaller or larger than the body, as it is nearer to or 

 farther from the lens than the body itself is. If the body is brought as 

 near to the lens as the distance of the focus for parallel rays, no image 

 would be formed, for the rays would be refracted parallel to each other ; 

 and if the body were brought still nearer, the rays would diverge after 

 refraction. The case of a convex lens is one of the most simple and the 

 most important ; but the same principle may easily be extended to other 

 cases. Fi 27> 



We shall next explain the re- 

 fraction of a triangular piece of 

 glass, called a prism (jig- 27). 

 The sides are flat; it cannot 

 therefore bring the rays to a focus, 

 nor can its refraction be similar to 

 that of a flat pane of glass, because c , (i ^ ] 



it has not two sides parallel. The 



refractions of the light, on entering and on quitting the prism, are both in 

 the same direction"!". On entering the prism P, the ray is refracted from 

 B to C, and on quitting it from C to D. If the window-shutters be 

 closed, and a ray of light, admitted through a small aperture, fall upon a 

 prism, it will be refracted, and a spectrum, A B (Jig. 28), representing 

 airthe colours of the rainbow, will be formed on the opposite wall. It is 

 difficult to conceive how a piece of white glass can produce such a variety 

 of brilliant colours ; but the fact is, that the colours are not formed by the 

 prism, but existed in the ray previous to its refraction ; for the white rays 

 of the sun are composed of coloured rays, which, when blended together, 

 appear colourless or white. 



Sir Isaac Newton, to whom we are indebted for the most important 

 discoveries respecting light and colours, was the first who divided a white 

 ray of light, and found it to consist of an assemblage of coloured rays, 

 which formed an image upon the wall, such as is exhibited (Jig. 28), in 

 which are displayed the following series of colours red, orange, yellow, 

 green, blue, indigo, and violet. Now a prism separates these coloured 

 rays by refraction. It appears that the coloured rays have different 



* Wo speak of the formation of an image at a point wherever the rays after reflection 

 or refraction proceed, as if they diverged from that point. The object is then seen as 

 an image of it placed there would lie. This image, however, has not, under common 

 circumstances, any real existence. The rays pass through the point in question ; Init as 

 they are only seen by an eye in the direction of their motion, a spectator any where else 

 will not see them at all, and the spectator who does see them will only know the direction 

 in which they move. If, however, a screen be placed at the focns, so as to intercept and 

 reflect the rays, the existence of the image will be proved by the actual formation of a 

 distinct picture of the object upon the screen ; if the screen be placed nearer or farther 

 off, so that the rays have not yet accurately converged to, or have begun to diverge from, 

 the focus, there will be a contused spot of light, but no distinct representation. 



f This will at once appear, as in the case of the lens, by drawing perpendiculars to the 

 surface of the prism where the ray enters and quits it. 



