584 BIOLOGICAL EFFECTS OF RADIATION 



The lens in the eyespot serves to bring the shorter wave of light to a 

 focus on the photosensitive substance. This substance is much more 

 sensitive in the central part of the cup than elsewhere. This conclusion 

 rests on calculations based upon the relation between the location of the 

 focal point and the direction of the incident light, and the relation 

 between the direction of locomotion in a field of light (consisting of two 

 horizontal beams which cross at right angles) and the relative intensity 

 of the two beams. If photopositive colonies are exposed in such a field 

 of light, they swim toward a point between the beams. The location of 

 this point depends upon the relative intensity of the two beams. When 

 the colonies are oriented, the illumination on opposite sides is unequal 

 but the effect is equal. This is due to difference in the location of the 

 focal point in the eyespot on opposite sides. By ascertaining, therefore, 

 the location of these points in relation to the relative intensity of the 

 beams, the distribution of sensitivity can be calculated (Mast, 152a; 

 Mast and Johnson 161). 



Volvox is usually positive in weak and negative in strong light but 

 the reverse obtains under some conditions. It may be either positive or 

 negative or neutral in every condition of illumination in which orientation 

 occurs. If it is positive, a shadow on the photosensitive substance in 

 the eyespots in the zooids causes change in the direction of the stroke of 

 the flagella of the zooids from diagonal to backward, and a flash of light 

 on this substance causes change from diagonal to sidewise ; if it is nega- 

 tive, the reverse obtains; and if it is neutral, there is no response unless 

 the changes in luminous intensity are great. 



Reversal in the direction of orientation from positive to negative is, 

 therefore, due to internal changes of such a nature that shock-reactions 

 which were produced by decrease are produced by increase in the illumi- 

 nation of the photosensitive substance; and reversal from negative to 

 positive is due to the reverse. The nature of the response to light in 

 Volvox depends upon the state of adaptation and the intensity of the 

 illumination. If it is fully adapted in a given intensity, it becomes 

 positive if the intensity is increased and negative if it is decreased. If 

 it is not fully adapted, it becomes negative if the intensity is increased 

 and positive if it is decreased. 



The time required for colonies of Volvox to become negative or posi- 

 tive after the luminous intensity has been changed (the reaction time) 

 depends upon the degree of adaptation and the extent of the change. If 

 colonies which have been in strong light 1 to 2 hr., followed by a variable 

 period in darkness, are subjected to strong light, the time required to 

 become positive (reaction time) increases with increase in the length of 

 the period in darkness (dark adaptation) from 0.04 min. (with 2 min. in 

 darkness) to a maximum of 0.52 min. (with 16 min. in darkness) and then 

 decreases to 0.18 min. (with 25 min. in darkness). If the colonies^ are 



