332 LIGHT AND WEBER-FECHNER LAW 



animals eventually reach the top, thus proving the geotropic response 

 to be the major factor in orientation. Table I shows the retarding 

 effect of illumination from below. Incidentally these figures well 

 illustrate the constancy with which any given lot of animals responds. 

 It is apparent from this result that while light exercises a directive 

 action, the directive effect of gravity is greater than that of light of 

 the strongest intensity used. 



Since the geotropic response is shown by the beetles only when 

 illuminated, it therefore follows that their movement in a lighted 

 field is the result of three factors; viz., negative geotropism, photo- 



TABLE I. 



Light Intensity 3,276 Candle Meters* 



* With the light above, gravity and light act together to produce a response 

 in 10.4 seconds. When the light is below, the directive action of the light acts 

 against the effect of gravity and thus delays the response to 16 seconds. 



kinesis, and positive phototropism. The first is constant, the other 

 two factors are functions of the intensity of the illumination. In the 

 case in which the light is from above, it follows that changes in the 

 rate of movement of the beetles depend on variations in the degree of 

 photokinesis and of phototropism. These factors in turn depend 

 upon the intensity of the illumination. Hence we may express the 

 relation of the rate of response to the intensity of the illumination by 

 saying that the rate of response is a function of the light intensity. 

 In the present study, the determinations of reaction time were 

 made as follows. A cage of wire gauze 12.5 cm. high was constructed, 

 having a movable top of the same material. During a series of 



