480 DISPERSION OF LIGHT. 



the path of the rays B h i and B e k] thus a b is the image 

 of AS. 



The reduced virtual image produced by a concave lens can only be 

 seen by looking through the lens at the object, in the same manner 

 in which the magnified virtual image formed by a convex lens is 

 seen, when object and image are on the same side of the lens. The 

 image is the more reduced, the smaller the focal length of the lens. 

 Spectacles used by short-sighted persons are concave lenses ; the 

 images which they produce appear to a good eye smaller than the 

 objects, especially if the lenses are made for very short-sighted eyes, 

 that is, if they are very concave. The focal length of spectacle 

 glasses is usually a few decimetres, so that the image is not very 

 much reduced. For the experiments which are hereafter to be 

 made with reference to the explanation of the action of the telescope, 

 a concave lens of 6 cm focal length is required, which produces an 

 image considerably smaller than the object. 



41. Dispersion of Light. The Spectrum. The ap- 

 pearance of colours has been already alluded to in de- 

 scribing the action of the water prism in the preceding 

 article ; the same phenomenon is also rendered visible, 

 although in a smaller degree, by a glass lens. A pencil 

 of light which has passed through a prism does not 

 form a white spot upon a screen, but one striped with 

 colours; objects viewed through prisms appear with 

 indistinct and coloured outlines ; images produced by 

 lenses have similar coloured fringes, although the 

 coloured part is usually narrower than in the case of 

 prisms. The cause of this is that common white light 

 is a mixture composed of light of various colours, 

 each colour being refracted differently from any 

 other. The greater the deviation from their original 

 path of the rays which have passed a prism, that is, the 

 greater the refracting angle of the prism, the more 

 separated are the different colours : the band of colours 

 is wider if the prism has a refracting angle of 20 than 



