SELIG HECHT 155 



light and the sensitization period should be expressed by the re- 

 ciprocity law as found by Bunsen and Roscoe (1862). This states 

 that for a given photochemical effect (E) a definite quantity of 

 energy is required, and this same result is obtained whether the in- 

 tensity (/) is high and the exposure (/) short or conversely. In 

 other words, 



E=k-I-t (1) 



The applicability of this law has been demonstrated by Blaauw 

 (1909) and by Froschel (1909) for the photic reactions of some plants, 

 and by Loeb and his coworkers for the orientation of several species 

 of animals. In Ciona, its existence would be demonstrated if the 

 product of the intensity and the sensitization period were a con- 

 stant quantity. In Column 4 of Table II are given the values ob- 

 tained. It will be seen that the figures agree well with one another 

 and with the average value. 



The matter may be illustrated graphically. The equation repre- 

 senting the Bunsen-Roscoe law may be written 



E 



— = k- I 2) 



t 



In this form, it represents the equation of a straight line. Since, in 



E 

 Ciona, E is constant, — may be represented by the reciprocal of the 



sensitization period, and the relation between this quantity and the 

 intensity should be represented by a straight line. Such indeed is the 

 case, as shown by the position of the points in Curve B of Fig. 1. 



3. In terms of the processes that go on in the sense organs, the 

 applicability of the reciprocity law means that during the sensitiza- 

 tion period the light must form a constant quantity of a substance 

 before it can produce its stimulating effect. Undoubtedly this in- 

 volves the conversion of a photosensitive substance into something 

 else which acts as an "inner stimulus." Many such photosensitive 

 substances are well known and have been studied with considerable 

 care (Sheppard, 1914). 



With a more precise presentation of the dynamics of the photo- 

 chemical reaction in Ciona we shall deal in a following section. At 

 present, however, I wish to emphasize a corollary to the fact that the 



