Wager: Movements oj Aquatic Micro-Organisms. 209 
along a resultant path between the two sources of light they 
will tend to accumulate in the space immediately adjacent to 
and between the two sources of light. But if the Euglena; 
are attracted by a stronger light it is clear that all those upon 
the a side of 1 — i 1 will be attracted to a, and all those upon 
the b side of 1 — i 1 will be attracted to b, simply because those 
on the a side are in a region of more intense light as regards 
a than b, and those on the b side in a region of more intense 
light as regards b than a. owing to the operation of the law 
of inverse squares. The Euglenae will then accumulate in 
two equal masses at a and b as shown in Fig. XL 
Similarly, if the light at a is twice as strong as at b then the 
curved line 2 — 2 1 marks off the area of influence of each ; all 
the Euglenae on the a side of 2 — 2 1 will move to a ; all those 
on the b side to b. In the same way when the ratio of a to b 
is as 3 to 1, or 6 to 1, or 12 to 1, the curved lines 3 — 3 1 , 6 — 6 1 
and 12 — 12 1 mark off the areas of influence of a and b re- 
spectively, and the Euglenae will accumulate at a and b 
approximately in proportion to these varying light intensities. 
This explanation does not, of course, apply when the light 
intensity is so strong at one of the sources as to bring about 
repulsion ; in that case a larger accumulation of Euglenae 
would take place at the weaker light. 
The nature of the response is very much affected by light 
of different colours or wave lengths. Thus both Strasburger 
and Engelmann found that green swarm spores and Euglena 
tend to accumulate at the blue end of the spectrum. In a 
typical case Engelmann showed that about 70 per cent, of the 
Euglenae were attracted by the green-blue part of the spectrum. 
11 per cent, by the green, 16 per cent, by the blue-indigo and 
only 1.4 per cent, by the orange red. My own observations 
confirm these results. Many contradictory statements have 
however been made and some observers have maintained that 
these and similar organisms are more responsive to the green 
and yellow-green than to the blue. Thus Loeb and Maxwell* 
find that Chlamydomonas gathers in the green part of the 
spectrum and that the larvae of a barnacle ( Balanus per- 
joratus) accumulate in maximum density in the green, possibly 
towards the yellow green. 
Experiments which I made with spectra of varying degrees 
of purity showed that these results, which differ from those 
of other observers, are probably due to the use of an impure 
spectrum. The authors state that they tried both prism and 
* ‘ Further Proof of the Identity of Heliotropism in Animals and 
Plants.’ University of California Publications in Physiology, Vol. 3, 
1910. 
Cf. also Ewald, Jour. Exp. Zoology, 13, 1912, who comes to much the 
same conclusion as regards the nauplii of Bolanus perforatus. 
101 1 July 1. 
