274 OUTPOSTS OF THE INTELLIGENCE SERVICE 



(for a thin lens) its focal length. If the focal lengths are given in metres, 

 then their reciprocals give the power of the lens in dioptres. For example, 

 a lens with a focal length of 1 metre is of 1 dioptre : of 2 metres of 0-5 dioptre ; 

 of 0-3 metres of 3 dioptres, and so on. Convex lenses are positive, and their 

 power in dioptres is given with the sign -\-. The negative sign is placed with 

 the dioptric value of concave lenses. 



When a source of light, e.g. a candle, is placed near a biconvex lens, we see 

 very clearly two images produced by reflection. The first is formed by the 

 anterior convex surface and is upright, the second is formed at the posterior 

 surface, which is concave in respect of rays passing out of the lens into the 

 atmosphere, and is inverted. The size of the image decreases as the con- 

 vexity of the lens increases. Its brighf)iess increases the more obliquely 

 the rays from the candle strike the surface and also with increase of the 

 refractive index of the medium composing the lens. 



If we now replace the pinhole of the camera with a convex lens, we will 

 find that we still obtain an image of the arrow on the screen. The introduc- 

 tion of the lens brings advantages and disadvantages. Two of the chief 

 drawbacks are chromatic and spherical aberration. 



Chromatic Aberration. When white light, which is a mixture of waves of 

 various frequencies (q.v.), passes through a lens, each monochromatic con- 



FiG. 70. — Chromatic Aberration. 



stituent is refracted to a degree depending on its frequency. That is, the 

 refractive index of the lens has a different value for each type of light, and 

 therefore the different waves, when they strike the bounding surfaces of the 

 lens, will undergo different deviations. Those of high frequency, violet rays, 

 are refracted to a greater degree than the slower, longer red rays. 



In Fig. 70 the dotted lines represent the path taken by limiting parallel 

 violet rays, while the continuous lines coming to a focus further away from 

 the lens represent the paths of the red constituent rays of white light. If the 

 screen is placed as in the diagram midway between the principal foci for 

 violet and red rays, the image of the arrow will appear surrounded by a red 

 and violet lialo. 



Chromatic Difference of Magnification. This defect also is due to the 

 unequal refraction of waves of different frequencies. Not only do the foci 

 of the various monochromatic components of white light fall on different parts 

 of the principal axis, but their pencils form different angles with the optic 

 axis. Thus, rays entering the lens at a considerable angle come to a focus 

 at a point depending on the colour of the light and therefore the size of the 

 image produced will also depend on the colour of the light. That is, violet 

 rays will produce an image smaller than that produced by blue rays, blue 

 smaller than green, green smaller than yellow, while the largest images will 

 come from red rays. 



These defects may be overcome by the use of a combination of lenses. 



