

Examination of Photographic Lenses at Kew. 443 



4. But it must be confessed that this rule may have little or no 

 relation to the truth with a corrected lens, and it is merely adopted 

 as the only approximation obtainable. It is assumed, therefore, that 

 uw = 5/4 bw. Let bw, the result obtained by the examination for 

 achromatism, = a ; let the diameter of the disc of confusion, or twice 

 u'w, = '; let the principal focal length of the lens =/; and let the 

 effective aperture = e. Then it can be seen, by reference to fig. 6, 

 that 



* = * = 8V(C.I. No. of stop). 



e 



The table on p. 427, which gives the values of 20 a -/(O.I. No. of 

 stop), affords a ready means of obtaining the required results in the 

 following manner : 



Knowing the C.I. No. of the stop, decide on ', the diameter of 

 the maximum disc of diffusion that will be tolerated ; then, under 

 the columns thus ascertained, look out ft in the table, multiply the 

 figure there given by f, and the maximum difference of focus, a, 

 that can be tolerated between white and blue rays is thus obtained. 



Or, in the line opposite the stop of the size under consideration, 

 find a number equal to *, the observed difference of focus for white 

 and blue rays ; then 6', the diameter of the disc of diffusion, will be 5/2 

 times 6, the figure given at the top of the column in which a. has been 

 found. 



It may be observed that the either the principal focal length or 

 the position of the nodal point of emergence may vary as different 

 coloured lights pass through a lens. It would not be difficult to 

 investigate these two sources of error separately, but the results 

 would be of little or no practical value. 



16. Astigmatism. Approximate Diameter of the Disc of Diffusion in 



the Image of a Point, with stop G.I. No. at inches 



from the centre of plate = 0' inch. 



The following is the method of examination for astigmatism : 

 The room is darkened, and in front of the lens is placed a thermometer 

 bulb, thus obtaining, by means of the reflection of the light of a small 

 lamp, a fine point of light. The lens holder of the testing camera is 

 revolved upwards or downwards about the horizontal axis so that the 

 axis of the lens makes an angle, 0, with the path of the rays coming 

 from the thermometer bulb ; the angle is such that the point of 

 observation represents the extreme corner of the plate of the size of 

 which the lens is being examined; that is to say, if, in fig. 20, GG 

 represents the position of the ground glass, then CP is equal to 

 half the diagonal of the plate ; this angle has already been found for 

 previous tests. If the lens shows any astigmatism, the image of 



