RELATIVE STIMULATING EFFICIENCY OF LIGHT 143 



Northrup (1917) that it holds for Balanus larvae; Patten (1915) 

 that it holds for blow-fly larvae and Clark (1913), Blaauw (1909), 

 Froschel (1910), and Fitting (1905) that it holds for certain 

 plants. 



In these tests, as in those previously mentioned, the flash- 

 frequency was usually high. For example, in the work of Loeb 

 and Northrup the intermittent light was of a flash-frequency 

 of from 50 to over 83 per second, while Patten used a frequency 

 of interruption of 115 per second. 



Apparently Ewald alone has investigated the effect upon an 

 animal other than man of intermittent light of low flash-frequency. 

 He states (1914) that Daphnia orients in intermittent light as 

 it does in continuous light independently of whether the fre- 

 quency of interruption is 1 or 30 per second, but that, in refer- 

 ence to the orientation of the eye of Daphnia, intermittent light 

 of lower frequencies of interruption has a weaker effect than 

 continuous light. He says (1913, p. 237) : 



In some cases I got a marked reaction of the eye on change from 

 constant to intermittent light of equal energy when the speed of the 

 sector-wheel was about one-tenth of a second per revolution. The 

 deviation becomes more marked the slower the speed. 



In harmony with the results described above are those obtained 

 in certain investigations on the effect of light upon photographic 

 plates. Abney, Kron, Helmic, and Newcomer state that the 

 blackening of photograpic plates by light does not depend solely 

 upon the total energy received. Helmic (1918, p. 374) main- 

 tains that it "is dependent upon the rate of flow of energy, with 

 total energy constant; and that for each brand of plate and quan- 

 tity of total energy there is a maximum blackening given by a 

 certain rate of flow of energy. " According to Helmic, Abney 

 (1901, p. 395) and Kron (1913, p. 755) reached the same con- 

 clusions. Moreover, Newcomer (1919, p. 243) asserts that in- 

 termittent light produced by a rotating sectored disk with a 72 

 degree aperture produces less blackening of a photographic plate 

 than continuous light of equal total energy. He does not state 

 the flash-frequency used but it was presumably low. 



