556 Dr. Q, Majorana on the Relative 



measurement may have been carried out ; since the sun's 

 brightness varies with the season and the time of day, and 

 depends on the amount of atmospheric absorption. It is 

 evident that the variableness in the clearness of the atmosphere 

 affects the two quantities under comparison in the same sense, 

 leaving their ratio unaltered. 



The experimental determination of this ratio not only 

 renders unnecessary the use of any artificial source of light, 

 but with proper arrangements furnishes an extremely sensi- 

 tive method of studying the variations in the luminosity of 

 the sky. 



The investigation was first suggested by Lord Kelvin, during 

 his stay at Rome, in April 1899. It presents a difficulty 

 which occurs in almost all photometric determinations — that 

 of having to compare two sources of different colour. For 

 this reason, the measurement is essentially somewhat un- 

 certain. Nevertheless, it is possible by a properly arranged 

 apparatus to study fairly conveniently the grosser variations 

 in the luminosity of the sky. 



Measurements more precise than those about to be described 

 have already been carried out by others, but the scope of the 

 present research is limited to the observation of the influence 

 of the variations in the clearness of the sky on the light 

 emitted by it. Now in order to carry out observations in 

 an atmosphere as free as possible from water-vapour, though 

 somewhat rarefied, it becomes necessary to ascend high 

 mountains, and in that case the simplicity of the observations 

 becomes of paramount importance. 



Lord Kelvin advised the construction of an apparatus con- 

 sisting of two tubes, one of which w r as to be turned towards 

 the sun and the other towards the sky, each tube having an 

 aperture at one end and a paper screen at the other. By 

 altering the dimensions of the apertures and the lengths of the 

 tubes, it would be possible to adjust matters so as to get equal 

 illumination of the two screens, the observer viewing the latter 

 by covering his head with a black cloth. A knowledge of the 

 lengths of the two tubes and of the diameters of their aper- 

 tures suffices for the deduction of the ratio of the two luminous 

 intensities — that of the sun, and that of an equal angular area 

 of the sky. 



The arrangement proposed by Lord Kelvin, though ex- 

 tremely simple, is not convenient when it is desired to make 

 observations on mountains. Suppose, in fact, that the tube 

 which is to be turned towards the sky is made about 50 cm. 

 long, and has an aperture of about 5 cm. in diameter. In 

 order to obtain equality of illumination of the two screens, 



