202 LIGHT AND PROTOPLASM [Ch. VIJ 



action of the different rays of which it is composed. We 

 must now answer the question : What different effects do the 

 different rays have ? 



The effect of the different rays in phototaxis is very clearly 

 seen in the various groups of Protista and among the Fla- 

 gellata and the swarm-spores, there is entire uniformity of 

 response according to the testimony of Cohn ('65, p. 36), 

 Stkasbueger ('78, pp. 593-599), Engelmann ('82°, p. 398, in 

 Euglena), and Veeworn ('89^ p. 49, in Navicula). Here the 

 more actinic rays with shorter and more rapidly vibrating wave, 

 act exactly like white light, whilst the rays from the opposite 

 end of the spectrum have no more effect than darkness. More 

 precise determinations were made by Strasbukgee ('78, p. 

 597), who found that the swarm-spores of the alga Botridium 

 responded to the blue and violet, but especially to the indigo, 

 whilst the green and ultra-violet were alike without effect. 

 And Engelmann, by means of his microspectral apparatus, 

 was able to determine that Euglena responded chiefly to the 

 rays X = OAlfi to X =0.49 fi ; that is, rays very near Featjen- 

 hopee's line F. The colorless Myxomycetes agree with the 

 chlorophyllaceous forms, according to Baranetzki ('76, p. 

 332), in responding to blue rays only. 



Among the higher organisms, Hydra, according to Wilson, 

 accumulates especially behind blue glass, to a small extent 

 behind green glass, and is entirely indifferent both to the upper 

 violet rays and those below the green. The photophil starfish 

 Astracanthion rubens, even when deprived of its eyes, was 

 found by Grabee to be "cyanophil"; even, though in slight 

 degree, to a low intensity of light. Among Mollusca, Grabek 

 found that the photophil Rissoa moved towards the blue even 

 when the intensity of the blue light was less than that of the red, 

 and Driesch asserts that the photophob Littorina rudis shuns 

 only blue rays. Thus, without multiplying cases, the results 

 of experiments may be summed up as follows : positively pho- 

 totactic or positively photopathic organisms are such only in 

 the presence of the blue rays. 



There are some few observations which are in apparent dis- 

 cord with this conclusion. Whether the ultra-violet rays are 

 ever active is a fairly debatable question. Lxjbbock ('82, 



