28 BOTAMCAL GAZETTE [july 



A quantitative measurement of the greater effectiveness of one 

 •spectral region over another of equal intensity might be measured' 

 by the angular deviation of the path of a motile organism from 

 a line perpendicular to a line connecting the two sources. 



All these methods, with the exception of those of Kniep and 

 Minder, involve special apparatus often not easily available. 

 For many problems simpler methods will accomplish the same 

 ends. For the present investigation a method has been devised 

 whereby color screens of known wave length transmission are used, 

 and their relative intensities measured by means of a thermopile 

 and galvanometer, and made equal by adjusting the distances from 



the light source. 



Apparatus and methods 



As biological science becomes more exact, with the tendency 

 to reduce the expression of natural phenomena to mathematical 

 formulae, it is obviously essential to define stimuli of all sorts 

 quantitatively. Indefinite or incomplete records of light stimuli 

 can no longer be attributed to the lack of means of measuring 

 them, because access to a spectroscope and thermopile makes it 

 possible to analyze any light qualitatively and quantitatively. 



There are two methods of obtaining monochromatic light for bio- 

 logical experiments, namely, the projection of a spectrum upon the 

 organisms, or the use of filtered light passed through a color screen. 

 The former is theoretically the better for exact work, but tech- 

 nical difficulties, such as limited dispersion and low intensity, 

 make it impractical for many investigations. Light filters of glass 

 are the most convenient means of securing approximately mono- 

 chromatic light when unilateral illumination is desired. Ordinary 

 color screens transmit too wide a range of wave lengths for exact 

 w r ork, and at present there are very few whose light is of sufficient 

 homogeneity. The best is the Wratten filter screen, which con- 

 sists of a dyed gelatine film betw r een two glass plates. MacDougal 

 and Spoehr have described some colored glass screens designed by 

 them for biological w r ork, but the range of wave lengths to which 

 each of them is transparent is considerably greater than for some 

 of the Wratten filters. 



