-v> u 3'. 



There are two methods for obtaining monochromatic light for 

 biological experiment/S ' . ---the projection of a snectrum upon 

 the yijH/y/i^X organisms or <s of filtered li r -'ht passed through 

 a colo screen. The former i .3 t"ieo etically the tHMrtr for exact ir 

 work but technical difficulties such as the limited dispersion 

 and low intensity make it impractical for 7iany investigations. 

 Light filters of glass are the :r.ost convenient "leans of se- 

 curin,- approximate! oc r ; I LC li't when unilateral ill- 



umination is desired. Ordinary color screens transmit too wide 

 a range of wave lengths for exact T .vork and at present there t 

 are very f w iv';ose lir'it is of sufficient homogeneity .The uest 

 ia the Wratten filter screen, made in London, <"hich consists o 

 of a 1 ' -al-'tine film between two p/lass platesVacDougal and 

 Sr>oehr(l9l7) have described some colored glass screens designed 

 by them for biological work, but the range of wave lengths to 

 which they ar-e transparent is considerably greater than for 

 the Wratten filter. 



In the experiments to be described, seven Wratten light 



filters were used , each of which was fitted as a window in the 



fy 



end of a dak b .x. i-lach transmitted only a narrqw range of 



-ave lengths but all together they embraced the whole of the 

 visible spectrum. The wave lengths to which each screen was 

 transparent ^ev* determined by testing tie lipht transmitted by^ 

 each with a direct wtsion spectroscope with a wave length scale 

 attached. Thus the quality of the light stimulus acting in 

 each box is accurately known. 



