452 PHOTOREACTIONS OF WHIP-TAIL SCOE,PIONS 



upper end of the deflection range. It has been shown (1) that the 

 increase in illumination necessary to produce an initial deflection of 

 definite value is less than the increase in illumination necessary 

 to produce an increase in deflection of hke value. The data col- 

 lected were not sufficiently extensive to justify the statement 

 that the increases in deflection followed mathematically the Weber- 

 Fechner law, but the curve of increase in deflection with in- 

 creasing lateral illumination is certainly of that general type. Ap- 

 plied to the data under consideration, this means that the first 10° of 

 deflection are more readily induced than the deflection from 10° to 20° 

 and so on. A reduction in deflection to 50° from a normal of 60°, 

 following interference with receptors, does not indicate therefore that 

 the receptive mechanism has been reduced by one-sixth in efficiency. 

 On the contrary, it would require a reduction of considerably more 

 than one-sixth in photoreceptive effectiveness to reduce by one-sixth 

 the normal reaction. 



While it is not possible, in the light of the above considerations, to 

 compute the effectiveness of the different receptors on the basis of 

 the percentage reduction of normal reactions produced by their elimi- 

 nation, their relative effectiveness may, nevertheless, be deduced from 

 the available data. All the induced reductions in reaction, because 

 of the method of measurement employed, fall in the same part of the 

 deflection range. They can, therefore, justifiably be compared with 

 each other. This comparison has been worked out in Table VII by 

 taking the effect of the elimination of the median eyes as unity and 

 comparing with it the effect produced by elimination of the other 

 receptors. By averaging' the values thus obtained under the three 

 different conditions of illumination used, we can approximate the rela- 

 tive effectiveness of the photoreceptors as median eyes : lateral eyes : 

 cutaneous areas :: 1 : 1.6 : 2.2. 



^ In computing the averages the responses to anterior illumination made by 

 animals with their lateral eyes capped were not given equal weight with those 

 made under lateral illumination. The reasons for regarding the experiments un- 

 der anterior illumination as less accurate as far as indicating the effectiveness of 

 the lateral eyes is concerned, are given in the text. 



