148 S. O. MAST 



(3) of the same intensity and form but differing in size. The spe- 

 cial relation of the two sources of light can be reversed and the 

 intensity accurately controlled and measured. 



The spectral color apparatus is designed for the investigation 

 of problems concerning color. By means of it animals can be 

 subjected, (1) to monochromatic light of various intensities and 

 exceptional purity from any region of the spectrum, (2) simul- 

 taneously to two beams of light of precisely the same wave- 

 length but of different intensities, and (3) simultaneously to 

 two beams of light of different wave-lengths but either of pre- 

 cisely the same or of different intensities. The light in these 

 beams can be selected from any two regions in the spectrum. 

 Their relative position can be readily and almost instantaneously 

 reversed. And the intensity can be accurately controlled, 

 measured and compared in terms of energy. 



It would be superfluous to attempt even a general descrip- 

 tion of either of these two pieces of apparatus. The monograph 

 should be in the hands of every one in any way interested in 

 reaction to light either in animals or in plants, perhaps not even 

 so much owing to the discussion bearing on these special pieces 

 of apparatus as to that bearing on the nature and experimental 

 value of light artificially produced in various ways and that 

 concerning the nature and production of colors by means of 

 screens, filters and reflecting surfaces of various sorts. 



