Luminous Intensities of Sun and Sky. 



557 



it is necessary to make the solar tube about 1"5 metres long, 

 and its aperture only *1 to "2 mm, in diameter. Hence, 

 owing to the inconvenience o£ making the solar tube very 

 long, there is an uncertainty in the determination of the ratio 

 of the two apertures. 



It was therefore preferable to construct a small apparatus — 

 a sky-photometer — easily portable, and enabling observations 

 to be taken rapidly. This is represented in fig. 1. The two 



Fitr. 1. 



tubes Tj and T 2 are intended to be turned towards tbe sun' 

 and sky respectively. They are rigidly fixed to two hollow 

 cylinders, Ci and C a . These cylinders are coaxial, and C 2 is- 

 capable of rotating respectively to C l9 and of being fixed in' 

 any desired position by means of the screw R. The cylinder 

 C 2 carries a third tube T 3 , which serves as the ocular tube. 

 The tube T, is at one end furnished with a small short-focus 

 lens L, in front of which is placed a diaphragm with an aper- 

 ture about 1*5 mm. in diameter. The divergent light coming 

 from the lens passes through two ground-glass screens V in 



