496 S. O. MAST 



although it appears to be somewhat nearer 493 mm for the posi- 

 tive than it is for the negative specimens. From this region the 

 stimulating effect decreases in either direction at nearly the 

 same rate for both. The distribution of stimulating efficiency 

 is also essentially the same for both and much Uke the distribu- 

 tion of the stimulating effect. The maximum efficiency is, how- 

 ever, somewhat nearer the violet, being near wave-length 483 mm, 

 and the efficiency of the shorter waves in general is relatively 

 higher than the effect. 



Loeb and Wasteneys ('16) in an interesting series of experi- 

 ments located the region of maximum stimulating effect for 

 Euglena viridis in a ' 'carbon arc spectrum" between wave- 

 lengths 460 and 490 ixix. This region includes the much narrower 

 region which we have established for this form, but the results 

 obtained by Engelmann ('82) in a very thorough study of the 

 behavior of the same species are considerably more nearly in 

 harmony with ours. 



Engelmann maintains (p. 398) that in a prismatic micro-spec- 

 trum thrown vertically upon a hanging drop under a microscope 

 with the spectrometer slit 0.42 mm. or less wide, a large majority 

 of the organisms collects in a narrow band in the blue between 

 wave-lengths 470 and 490 mm and that this result is the same with 

 strong gaslight (Sugg's Brenner) as it is with daylight used as a 

 source of illumination. With the spectrum divided into six 

 fields of equal width he gives the results obtained in one experi- 

 ment as follows and maintains that they were essentially the 

 same in all of many other experiments performed: 



Feld 1 (A bis C| D) : 2 Individuen. 

 Feld 2 (Cf D-Di E) : Individuen. 

 Feld 3 (Df E-EI F) : 16 Individuen. 

 Feld 4 (Ef F-Ff G) : 100 Individuen. 

 Feld 5 (F^ G-G) : 24 Individuen. 

 Feld 6 (G-Gi H) : 3 Individuen. 



The individuals in fields 3 and 5, Engelmann says, were close 

 to the edges of field 4, nearly all of them between wave-lengths 

 490 to 500 and 470 to 460 mm respectively. According to this 

 statement, field 4 was located between 470 and 490 ij.}x. This is, 



