DISTORTION DUE TO LENSES 275 



convex and concave, made of substances of different refractive index, or by 

 the insertion of colour filters to cut out a series of frequencies. 



Spherical Aberration. Eays of light passing through the peripheral part 

 of a lens are refracted more than those passing through the central part. 

 This tends to prodixce distortion of the image at the periphery. 

 . Curvature of Field is found in all convergent lenses of simple formula. For 

 example, the lines on a piece of squared paper examined through a positive 

 lens of about 1 or 2 inches focus will appear definitely curved, all except the 

 two lines at right angles to one another occupying the centre of the field of 

 vision. Pliotographs taken with the iris diaphragm wide open, if only a single 

 lens is used, will exhibit this aberration to a marked degree, e.g. sides of 

 buildings, etc., will appear curved, each part of the picture will be confused, 

 being formed by pencils refracted through various parts of the lens. 



Spherical aberrations may be avoided by the employment of a lens whose 

 curvature gradually decreases from centre to periphery, and by placing an 

 iris diaphragm or stop in front of the lens to cut off peripherally incident 

 rays. 



For a lens (of crown glass) to produce absolutely the smallest possible 

 amount of aberration it should be biconvex, the radii of curvature of its 

 surfaces being in the ratio of 1 : 6, the more strongly curved surf ace facing the 

 incident rays. Such a lens is termed a crossed lens. Crown glass has a 

 refractive index of 1-5. If a glass with a ^u, = 1-6 (flint glass) were used the 

 side away from the incident light woidd be flat, i.e. the lens would be plano- 

 convex. 



Comma. Kays coming from a point source form, with some lenses, an 

 image with a fine tail pointing towards the optical axis — just like an illumi- 

 nated comma. This aberration is seen easily with the old-fashioned carafe, 

 which when filled with water acted as a lens. The distortion is due to a 

 difference in the position of the image produced by different zones of the 

 lens. If the lens obeys the sine law it is free from comma. 



The camera, whether of the pinhole variety or fitted with a good lens and 

 diaphragm, may produce images which suffer from defects not due to the 

 optical system, e.g. halation, flare, irradiation and scattered light. 



Halation is noticeable only when the photographic plate or screen is 

 sutticiently thick for the image formed on the surface to be reinforced by a 

 reflection of the image from the internal glass surface, so producing a blurred 

 outhne or halo. Photographic films, owing to their thinness, do not exhibit 

 halation noticeably. 



Flare is due to the illumination of the screen by light reflected from the 

 internal surfaces of the compound lens. It is least when the refractive indices 

 of the heterogeneous optical medium are almost identical and when the light 

 passes through the lens along the optic axis. The greater the angle of 

 incidence of the rays and the greater the dift'erences in refractive index of the 

 components of the optical medium, the greater will be the possibility of 

 flare. 



Irradiation is the spreading of the image on the screen due to excess 

 illumination of the object. It is easily produced in jihotography, e.g. by 

 pointing the camera directly to the sun and getting apparently a " moonlight 

 scene." 



Scattered light is light reflected from the inner surface of the camera, e.g. 

 shiny bellows, badly blacked walls, etc. 



We will deal later with the ways in whicli the eye avoids these defects, 

 meanwhile we will consider the structure of the organ. 



18-3 



