SENSIBILITY OF VOLVOX TO SPECTRAL LIGHTS 353 



period is of the nature of a simple reversible chemical reaction 

 with increase of energy, taking place in a homogeneous medium. 

 The latent period, however, involves a catalytic reaction in which 

 the catalyst remains after the action of light, and is therefore a 

 reaction with loss of energy. 



There are two points in Hecht's work which we have not been 

 able to make clear to ourselves, the first of these concerns the 

 latent, the second the sensitization period, into which two parts 

 Hecht ('18, p. 152) divides the reaction time. Although he 

 repeatedly states ('18, pp. 162, 165; '19 a, pp. 547, 548) that under 

 given conditions of temperature and intensity the latent period 

 is constant, he makes the following contradictory remarks (Hecht, 

 '18, p. 153) : ''It will be seen that, within limits, the shorter the 

 exposure time (sensitization period) the longer the reaction time 

 and consequently the latent period;" ('19 b, p. 661): ''There- 

 fore the latent period also varies inversely with the duration of 

 the exposure." 



In the above quotations exposure time and sensitization period 

 appear to be synonymous, but in the following passage (Hecht, 

 '19 b, p. 659} a distinction is made: 



All the experiments agree in showing that for a given intensity the 

 reaction time varies inversely with the exposure for exposure periods 

 shorter than the sensitization period. Exposures for intervals greater 

 than the sensitization period make no change in the duration of the 

 reaction time. The sensitization period ma}'' thus be defined as the 

 minimum exposure necessary to produce the minimum reaction time. 



This distinction leads to confusion. Moreover, it is contra- 

 dictory to the Bunsen-Roscoe law, which Hecht ('18, pp. 155, 

 165, and fig. 1, p. 156) found applied to Ciona and later ('19 a, 

 p. 548) stated "has been shown to apply to such sensitization 

 processes." 



For a given intensity there can be but one presentation time. 

 This term is used instead of sensitization period to avoid con- 

 fusion. An exposure shorter than the presentation time is sub- 

 liminal. The striking similarity between the reaction-exposure 

 curve (Hecht, '19 b, p. 659, fig. 1) and the reaction-temperature 

 curve (Hecht, '19 c, p. 671, fig. 1) suggests a possible explanation 



