312 Lord Kayleigh on the Intensity of Light reflected from 



angle. The eye situated opposite to the edge C and looking 

 into the double mirror thus sees the direct and reflected images 

 superposed, so far as the different apparent magnitudes allow. 

 D represents a diaphragm and E a photographic portrait-lens 

 of about 3 inches aperture which forms an image of A and A! 

 on or near the plane F. At F is placed a screen perforated 

 with a hole sufficiently large to make sure of including all 

 the rays from A, A' which pass D. To determine this point 

 an eye-piece is focused upon F, so that the images of A, A f 

 are seen nearly in focus. Some margin is necessary because 

 the images of A, A' cannot (both) be accurately in focus at F. 



These adjustments being made, an eye placed behind F 

 and focused upon C sees the upper mirror illuminated by the 

 direct light (from A), and the lower illuminated by the 

 reflected light (from A'). And if the aperture at F is less 

 than that of the pupil of the eye, the apparent brightnesses 

 of the two parts of the field are in the same proportion as 

 would be the illuminations on a diffusing screen at C due to 

 the two sources. The advantage of the present arrangement, 

 as compared for example with the double-shadow method, lies 

 in the immense saving of light. In the case of water there 

 is a great disproportion (of about 50 to 1) in the illumina- 

 tions as seen from F. In order to reduce the direct light to 

 at least approximate equality with the reflected, Talbot's 

 device* of a revolving disk was employed. This is shown in 

 section at I, and in plan at I'. The angular opening may be 

 chosen so as to allow for the loss in reflexion, and for the 

 further disadvantage under which the reflected light acts in 

 respect of distance. The disk finally employed was of zinc, 

 stiffened with wood, and covered on both faces with black velvet. 



It was at first proposed to work as above described by eye 

 estimations ; but the necessity for a ready adjustment capable 

 of introducing small relative changes of brightness leads to 

 further complications. Moreover, the large disk which it is 

 advisable to use for the sake of accurate measurement of the 

 angular opening, cannot well be rotated at the necessary speed 

 of 20 or 25 revolutions per second. For this reason, and also 

 for the sake of obtaining a record capable of being examined 

 at leisure, it was decided to work by photography. This in- 

 volves no change of principle. The photographic plate H 

 simply takes the place of the retina of the eye. But now 

 the integration of the effect over a somewhat prolonged ex- 

 posure (of several minutes) dispenses with the necessity for a 

 rapid rotation of the Talbot disk, and allows us to obtain at 



* Phil. Mag. vol. v. p. 327 (1834). 



