560 Dr. Q. Majorana on the Relative 



i for that of the screen which is a partial representation of 

 the sky, then by means of such a photometer we find the 

 value of 



Let now our sky photometer be introduced between the 

 screen C and the source S (fig. 4), care being taken, by means 



Fig. 4. 



:w----------^ w 



<.-—cZ- 



of suitable arrangements, to prevent the light from S reaching 

 C. Further, C is placed sufficiently near the photometer, for 

 the reason already stated. Using the ocular tube, we adjust 

 the iris diaphragm until equality of illumination is obtained. 

 The reading n on the scale of the iris diaphragm is then noted. 

 The problem is to find the value of r corresponding to the 

 reading n. Now if a stand for the visual angle of the sun, 

 expressed in minutes, and d for the distance of the screen C 

 from the photometer, then 



/21600CV 



is the number of times which the area of contains a circular 

 area subtending an angle a at a distance d. 



If D is the distance of the source I from the photometer, 

 the quantity 



is m times greater than the required ratio. Hence to the 

 graduation, n of the diaphragm corresponds the value 



_ d?_ / 21G00 C \ 2 _ / 21600C \ 2 

 r ~ U 2 V Zirda / V 2tt*D )' 



As will be seen, this value is independent of the distance 

 of the screen C from the photometer ;. as should be the case 

 if the condition indicated by fig. 3 is satisfied. 



To each value of D corresponds a pair of values of n and 

 r ; these values may be used for constructing a calibration 

 table. 



The operation of calibrating the photometer presents no 

 difficulty, since the two sources C and S may be chosen so as 



