338 SCIENTIFIC AMUSEMENTS. 



be fringed with colours and so rendered indistinct, unless means are 

 adopted for correcting the fault and rendering the lenses achromatic. 

 For the same reason, the most refrangible actinic rays are brought 

 to a focus by a convex lens at points nearer to the lens than the 

 average of the visible rays, whilst the least refrangible heat rays 

 similarly form a series of foci slightly further off. This difference 

 renders it necessary to use perfectly achromatised lenses for photo- 

 graphic purposes, otherwise the picture formed by the action of the 

 actinic rays would be more or less blurred and imperfect, even when 

 the visible rays gave a clear image in the camera (Chapter XXV.). 

 In the case of the heat rays, the difference is usually sufficiently 

 glass " when exposed to the sun, so that some inflammable substance 

 is placed at the spot where the image of the sun formed by the 

 visible rays is fairly clear and distinct.* 



small to render a convex lens a more or less powerful "burning 

 Expt. 366. Formation of Images by Lenses. A concave lens 



behaves very simi- 

 larly to a convex 

 mirror in this re- 

 spect, that the image 

 is always virtual, 

 erect, and dimin- 

 ished. Thus fig. 

 185 represents the 



Fig. 185. Virtual Image formed by Concave Lens, virtual image, ab, 



seen by an observer 



at when looking through a concave lens at an object, AB, a 

 similar result taking place in all positions of the object as regards 

 distance from the lens. 



A convex lens, on the other hand, acts like a concave mirror, 

 the nature of the image differing according to the position of the 

 object with respect to the lens. When the object lies nearer to the 

 lens than the principal focus (or point representing the average 

 geometrical focus for all kinds of visible rays jointly) the image is 

 virtual, magnified, and erect, this position corresponding to the 

 use of a reading lens or simple magnifying glass (Expt. 367) ; when 

 at the principal focus, no distinct image at all is formed (i.e., the 

 image is at an infinite distance) ; and when further away from the 

 lens than the principal focus, a real inverted image is formed, 

 which is magnified if the distance of the image from the lens on 

 one side is greater than that of the object on the other, and 



* As in the case of spherical mirrors, lenses the curvature of which are 

 spherical are also; subject to spherical aberration, which introduces another 

 class of difficulties in constructing optical instruments. 



