558 Dr. Q. Majorana on the Relative 



succession, and is thereby weakened. By means of this 

 artifice, the illumination of the second ground-glass screen, 

 which is the one observed, is rendered much more uniform. 

 A mirror, S 1? reflects the light along the direction of the tube 

 T 3 . The tube T 2 does not carry a lens, but is provided with 

 a free aperture, and is fitted with an iris diaphragm D, which 

 serves to adjust the intensity of the light falling on the two 

 ground-glass screens similar to those in the tube Tj. A 

 second mirror, S 2 , provided with a central aperture, throws the 

 light coming along T 2 in the direction of T 3 . A ball-aud- 

 socket joint F enables the apparatus to be fixed in any position. 

 The instrument therefore allows of the tube Tj being directed 

 towards the sun, and the tube T 2 towards any other point of the 

 sky. The eye, by means of the ocular tube, thus sees an azure 

 field illuminated by the sky, and in its centre an elliptic spot of 

 a reddish or white colour due to the sun. The aperture of the 

 diaphragm D is adjusted so as to give equal illumination of 

 the two areas. 



The instrument described had to be graduated. For each 

 position of the iris diaphragm it was necessary to know the 

 ratio of the sun's luminous intensity to that of an equal 

 angular area of the sky (about 32'). The graduation might 

 have been effected by means of a comparison with the tube- 

 instrument suggested by Kelvin and described at the beginning 

 of this note. But such an operation would have been some- 

 what uncertain, involving the comparison of lights of different 

 colour. Instead of this, it is better to use a more indirect 

 method, involving the use of artificial sources of light of 

 known intensity, and depending on the following considera- 

 tions : — 



Let (fig. 2) the short-focus lens of the photometer be 



Fig. 2. 



represented by L, and the first ground-glass screen by 8. 

 Let the diameters of the lens and the screen S be about equal, 

 and let the distance L S equal several times the focal distance 

 of L. Let a source of definite dimensions, representing the 



