Lord Rayleigh on Pin-hole Photography. 99 



next, while (1) and (4) were decidedly behind. Thus we 

 may take as the most favourable aperture 2r = '026 inch. 



The mean wave-length of the light employed was found 

 with the aid of a grating by a comparison with a soda flame : — 



so that 

 Hence 



Mean X:\ D = 239: 226; 



v= *§? x 5'89 x 10- 5 =6-23 x 10" 5 centim. 

 22b 



. 2r 2 /l 1\ (-026) 2 7 2-54xl0 5 , _ Q 

 ^""TV + b ) = —T~U 6-23 =W9 > 



agreeing very well with what was expected from the curves. 

 If we now assume that the best value of y is 1*8, we can 

 calculate backwards from the photographic results to find 

 the mean \ there effective. We have 



10 



whence 



-*xl'52 = (2?Wi+ !)= ^=2\xl-8; 



X=4'2 x 10 " 5 centim., 



a little less than that belonging to Fraunhofer's line G. 



To test the improvement of definition which according to 

 (27) accompanies an increase of/, I have used an aperture of 

 *07 inch and a focal length of 7 feet. The aperture was per- 

 forated in sheet zinc, and was placed in the shutter of a room 

 otherwise completely darkened. The subject was a group of 

 cedars, and, being somewhat dark in the shadows, required an 

 exposure of about an hour and a half, even in sunshine. The 

 resulting 12 X 10-inch photographs fully bear out expectations. 

 To appear in natural magnitude the pictures would of course 

 need to be held at 7 feet distance from the eye ; but even at 

 3 or 4 feet the apparent definition is sufficient. 1 have also 

 taken panoramic pictures with an aperture of '027 inch and 

 a focal distance of 12 inches ; but in this case there is nothing 

 that could not equally well be done with an ordinary portable 

 camera. 



Terling Place, With am, Essex, 

 Dec. 2, 1890. 



