98 Lord Rayleigh on Pin-hole Photography. 



largest aperture gives the best image, but at a given distance 

 beliind the case is otherwise. For example, when the image 

 was formed at 8 inches distance, (2) and (3) were about equal 

 as respects the double hole, while (1) was decidedly inferior, 

 and that not apparently from want of light. In the case of 

 the grating (3) had perhaps the advantage over (2). 



A photograph of the double hole was now taken under the 

 same circumstances with an exposure of 80 minutes. Aper- 

 ture (2) was here decidedly better than (3), and (1) was 

 almost as good as (2). The (negative) image formed by (5) 

 exhibited a pair of white spots near the centre of a patch of 

 black, corresponding to the state of things indicated in th*> 

 curve y = 4:ir. The difference between the photograph and 

 the result obtained by eye observation is readily explained by 

 the smaller effective wave-length in the former case. 



The difference just spoken of is intensified when the light 

 is white. In one experiment upon cloud-light a = 21 feet, 

 b = 10 inches. In the resulting photograph, obtained upon 

 an Ilford plate with an exposure of 30 minutes, the image 

 from (2) was decidedly the best. 



We may utilize the last result to calculate the relation be- 

 tween aperture and focus most suitable for out of door photo- 

 graphy. We have 



( 2 ") 2 G + 0= ( -° 240)2 (i + ro) 



= 10" 5 x 5-99 inches = 10" 4 x 1'52 centim. 



Thus, if a = go , as may usually be supposed in landscape- 

 photography, the most favourable diameter of aperture is 

 given by 



(2r) 2 //= lO" 5 x 6-0 inches = 10~ 4 x 1*5 centim., 



the first number being employed if r and / are measured in 

 inches, and the latter when the measures are in centimetres*. 

 If /=12 inches, 2r = '027 inch. If /=7x 12 = 84 inches, 

 2?' = '071 inch. 



The experimental determination of the best value of y 

 is more easily effected by eye observations. In order to 

 render the wave-length more definite an orange-red glass was 

 employed. With a = 18 feet, b = S inches, the image formed 

 by aperture (3) was judged to be decidedly the best, (2) was 



* The effect of varying the diameter of aperture in photographic land- 

 scape work has been tested by Capt. Abney ; but I am not in possession 

 of the data as to focal length necessary for a comparison with the above. 



