32-- 



orgaii^ms reacted positively at 40 cm. from the aro, negatively at 

 20 cm. In another, the neutral point seamed to be 50 cm. away. 

 Quite often Tit this distance there were nearly as raany colonies 

 swimming away from as towards light. The response wis always more 

 definitely positive at 100 om. than at 50 era. Sometimes, however, 

 the response was mrstly positive as close as 10 om. , so that the re- 

 versal is dependent to a large extent en the -cndition of the org'-n- 

 isms as for the other screens, the greatest intensity obtainable be- 

 hind any was too low to produce any negative renuonse. 



An entire carbon aro run was projected on one dish of 



organisms. By looVing down it could be seen immediately that while 

 those crT.nir"3 fin din selves in the yellow, ^reen, blue and 



violet lip;ht swam LowirdH the litf-.t, these of the red waved not to- 

 wards the front of the dish but in a direction por.^ii^i to it, i.e. 

 towards the yellow. The yellow apr.eared to the eye the Brightest 

 part of the dish. The organisms in it were dcubtless decidedly 

 photo vronic boc-'i-se of the relatively greater intensity, while those 

 of the blue were so in almost equal degree because cf the greater 

 ef f ectiveneflr* of these wave Tr-n;-:tr.8 even ^ t low intensity. In the 

 red the intensify wna not gre t en< ugh U) overcome the ineffectiveness 

 of the lon/3 r;iys, 30 tiie organisms responded either to an intensity 

 gradiant whi oh led them towards the yellow, or to > wave length 

 gradiant which would also lead tnen toward the yellow. Or the r - 

 suit might have be-=>n due to a diffusion of the more ei-:irint rays 

 into the red. The rara a red rayp in th< 5 naVed aro exposure thre 

 was no -iefl -ntion of the pat!"! leading dire^t-ly toward? the source of 

 light. 



