('?" J 

 Laurens in an investigation of the reactions of amphibians 



used these sane metnods and the same ^ppnratus for the quanti 

 tave analysis oi - the monochromatic light he u-ed^ ard for bnlanc- 

 ing then with respect to their relative intensity. 



Gross! Ifi2) also used these snrne methods in determining 

 the reactions of Arthropods to raonocnromatic light. 



An instrument has been devised by I'.acDougal and Gpoehr. ( ) 

 which ner-sures the total radiant energy of any light in terras of 1 

 its dissociation effect on a photosensitive substance. This is 

 measured by a galvanometer, "he advantages in the use of this 

 "photoelectric cell" are said to be its extreme sensitiveness t% 

 the wave lengths of the blue end of the spectrum, arid the fact thst 



its action in ligi-t is H raore nesrly that of the organism than that 



v 



of any othor light measuring instruments available. 



There have been therefore three different methods worked ot 

 out for biological experiraentsf or the quantitoive analysis ofi 

 light stimuli; viz., those of Kniep and Hinder, Day, and of 



^ racUoupiJ and Sooehr. The interesting apparatus of Patte 



add of I.oeb and Northrut) wherebya quantitave measurement of the re- 

 action of organisms subjected to two beams of light of different 

 intensity, is obtained. The measurement is in terms of the angular 

 deflections from an initial path of locomotion. The same methods 

 might be ar^olied to work with colored lights. A quantitative measur- 

 ment of the greater effectiveness of one spectral region over another 

 of equal intensity might be measured by the angular deviation of the 

 rath of a motile organism from a line perpendicular to a line connect- 

 inp the two sources. 



