426 



Major L. Darwin. On the Method of 

 FIG. 0. 



lens. The effect of this curvature is to make the image of a point 

 appear on the plate as a disc, except on the circle or at the point 

 where the principal focal surface either cuts or touches the plate. 

 If the photographic plate is in the position M'F', such that it bisects 

 cd, then the discs of diffusion will be greatest at the centre and at the 

 margin of the plate ; and any movement of the plate from this position 

 will increase the size of the disc at one or other of these places ; if the 

 photographer adjusts his focus so as to produce the best general focus, 

 eg (= t) will therefore be the diameter of the largest disc of diffu- 

 sion on his plate,. Assuming that this position of the plate has been 

 adopted, and that the lens gives no distortion, then, by similar 

 triangles, it can be seen that : 



= .(f-ri= nearly 



(1). 



But the C.I. No. of stop = -&-- ; and therefore 



-LOU 



A* = 20-/(C.I. No. of stop) (2). 



The following table gives the value of for different values of ^ 

 and for stops of different numbers ; and thus the size of the greatest 

 disc of diffusion can at once be seen from the results of the examina- 

 tion as recorded in the certificate. 



