Artificial Colour-blindness to Sicccessive Contrast. 



215 



shaft is so fixed that the slits of the smaller disc may revolve close in 

 front of the slit C of the spectroscope, an ordinary single-prism instru- 

 ment, fiu-nished with, a reflected-scale tube, the scale being removed, 

 leaving the tube open. A mirror placed at D in front of the sectors of 

 the larger disc reflects light from the sky on to a second mirror E, by 

 which it is reflected into the scale tube, causing the field of view to be 

 filled with a soft white light. 



Fig. 8. 



The mirrors and discs must be adjusted until on rotating the shaft 

 slowly the flashes occiu- in the following order : — First a sharp flash 

 through the small disc, giving a momentary view of the spectrum. As 

 soon as possible after this is over, but not before, there is a rather 

 long soft flash of pure white light, followed by a much longer period 

 of perfect darkness. The reason for having two slits and two sectors 

 on the discs is simply that they may be well balanced on the shaft, 

 and therefore rotate more steadily. There should be no overlapping 



