32 



BOTAXICAL GAZETTE 



[JULY 



A series of measurements for live second exposures agreed very 

 well with those obtained by the other method. 



The absolute intensities of the light behind the colored screens 

 were measured with a Sharp-Millar photometer. 1 Since at the 

 prescribed distances all are equal to the white light control, there 

 remained only to measure the intensity of the arc at the distance of 

 the control. For the experiments the distances obtained in the 

 calibration were divided by four, so that the intensity of the arc 

 with the photometer 85 cm. away was measured. It was 2050 meter- 

 candles. This must be corrected for the absorption of the light 

 by the glass of the filters. To get this " absorption coefficient/' 

 the intensity of a light was measured both with and without 

 a screen of clear glass equal in thickness to that of the filters. A 

 Lummer-Brodhun photometer was used for this determination. 

 It was found that glass 1 .5 mm. thick absorbed 12 per cent of the 

 light falling upon it. To obtain the intensity of the lights as actu- 

 ally transmitted by the light filters, it was necessary therefore to 

 take 88 per cent of the reading of the photometer (2050), which 

 was 1804 meter-candles. Of course the light stimuli acting on 

 organisms in water in the culture dishes were still less, owing to 

 absorption by the glass of the dish and of the water. 



TABLE I 



Distances at which intensities of light from an electric arc 



transmitted by wratten light filters are equal 



Filter no 



70 



71 



72 



73. . , 



74 



75 



76 



Control. . . 



Wave lengths in 

 Angstrom units* 





Color 



6600-7000 

 6200-6800 

 5900—6200 

 5600-5900 

 5200-5600 

 4700-5200 

 4000-4700 

 4000-7000 + 



Red 

 Red 



Orange 



Yellow 



Green 



Blue 



Violet 



White 



Distance from light in cm 



320-7-4 



275-5 

 230-^ 



250^ 



280 -S 



250 

 250 



340 



4 

 4 

 4 

 4 

 4 

 4 

 4 



80.0 

 68.7 



575 

 62.5 



70.0 



62.5 



62.5 



85.0 



Intensity in 

 meter-candles 



1804 

 1804 

 1804 

 1804 

 1804 

 1804 

 1804 

 1804 



* Professor E. P. Lewis of the Physics Department of the University of California very kindly 

 made these wave-length determinations. 



The lack of agreement between these values and the energy 

 curve of the spectrum is probably due mainly to the peculiar 



1 1 am indebted to Mr. W. C. Pomeroy of the Physics Department of the Uni- 

 versity of California for these determinations. 



