Manchester Memoirs, Vol. Ix. (1916), No. 10. 3 



measurement is of log i,/i. In view of the possibility of 

 slight defects in the condenser A, the sector has been 

 constructed to rotate if necessary. This can be done 

 either by hand or by clockwork. 



Adjustment. A white disc bearing a small black 

 cross at its centre is set up in the position of the spark 5 

 and illuminated by a lamp. The preliminary adjustment 

 is done by means of screws on the bracket carrying 

 prisms E and F. When this is properly done, the eye, 

 placed at E, can see a circle of light bisected by a larger 

 semicircle. The eye is then moved back ten inches and 

 the other bracket screwed up until the upper portion of 

 the cross transmitted by the upper path is seen to form a 

 complete image with the lower portion of the cross trans- 

 mitted by the lower, or standard beam {Fig. 3). The 



Fig- 3- 



instrument is now in complete adjustment and has now 

 to be placed with the top prism E nearly touching the 

 middle of the slit and with the upper path in the optical 

 axis of the spectrograph. The spark 5 is placed in the 

 position of the illuminated disc. 



Advantages. In all other methods of ultraviolet 

 photometry the intermittent nature of the light makes a 

 correction necessary. Schwartzschild's formula is used, i.e. 



i/ix = 4/' M - 

 In the case of Wratten panchromatic plates 7i = cyg. In 

 the Hilger instrument this difficulty is to a certain extent 

 obviated by calibrating the plates in terms of a piece of 



