19 2o] HURD—FUCUS SPORES 27 



was used to determine the exact range of wave lengths passing 

 "through the slit in each of the four illuminations used (red, yellow, 

 green, and blue). Day measured the intensity of each with a 

 Boys radiomicrometer, and balanced them by varying the number 

 of glowers employed in the lamps. Thus there was one glower 

 for the red light, two for the yellow, and three for the green and 

 blue. 



Laurens (12), in an investigation of the reactions of amphib- 

 ians, employed these same methods and the same apparatus for 

 the quantitative analysis of the monochromatic light he used, and 

 balanced them similarly with respect to their relative intensity. 

 Gross (9) also used the same methods in determining the reactions 

 of arthropods to monochromatic light. Mast (14) measured the 

 different effects of monochromatic light by the orientation of 

 organisms in a field in which two differently colored beams crossed 

 at right angles. He used spectral regions of known energy and 

 wave length. The method of obtaining the distribution of energy 

 is not described. Parr (15) did quantitative work on the response 

 of Pilobolus to different wave lengths, using apparatus of the type 



employed by Day and Laurens. 



An instrument has been devised by MacDougal and Spoeiir 

 (13) which measures the total radiant energy of any light in terms 

 of its dissociation effect on a photosensitive substance. This is 

 measured by a galvanometer. The advantages in the use of this 

 " photoelectric cell" are said to be its extreme sensitiveness to the 

 wave lengths of the blue end of the spectrum, and the fact that its 

 action in light is "more nearly that of the organism than that of 

 any other light measuring instrument available." 



There have been, therefore, three exact methods worked out 

 for biological experiments to obtain a quantitative analysis of 

 light stimuli, namely, those of Kniep and Minder, Day, and 



MacDougal and Spoehr. The interesting apparatus devised bj 



Patten (16), whereby a quantitative measurement of the reactions 

 of organisms subjected to two beams of light of different intensity 

 is obtained, might be mentioned here. The measurement is in 

 terms of the angular deflections from an initial path of locomotion. 

 The same methods might be applied to work with colored lights. 





