ZOOLOGY: S. O. MAST 
623 
purpose of showing that they are the same in both, hoping thus to 
demonstrate that there is nothing in the nature of psychic phenomena 
in animals. Other investigators, e.g.,Engelmann, Wiesner, Strasburger, 
Verworn, Parker and Blaanw, were interested in the problem more from 
the point of view of comparative physiology. 
I have for some time held the opinion that, aside from the importance 
of this problem in comparative psychology and physiology, it ought 
to yield results which would throw light on the nature of the chemical 
changes in the organisms associated with the reactions to light, espe- 
cially those associated with changes in the sense of the reactions. 
The chief difficulty encountered in the work has been connected with 
obtaining monochromatic Hght and measuring it in terms suited for 
comparison. Thus while a considerable number of organisms, both 
plants and animals, have been investigated in regard to the relation 
between wave-length and stimulation, only in a few are the results of 
such a nature that they can be compared with sufficient accuracy to 
warrant more than very tentative conclusions. Moreover, in a number 
of cases, otherwise excellent, only the region in the spectrum of maxi- 
mum stimulation has been ascertained. In the following observations, 
to be published in full elsewhere, these difficulties and defects have been 
to a large extent eliminated. 
It is well known that many of the simplest organisms respond very 
definitely to light. Some orient and travel fairly directly towards the 
light, others away from the light, while still others go toward it under 
some conditions and away from it imder others. 
In a field of light consisting of two horizontal beams crossing at right 
angles thesfe organisms proceed toward or from a point situated between 
the two beams. The location of this point depends upon the relative 
effective illumination received by the organisms from these beams. If 
it is the same in quality and quantity, so that the stimulation is the 
same, the point Hes approximately half w^ay between them. (Details 
as to the process of orientation in these organisms may be foimd in my 
book Light and the Behavior of Organisms, John Wiley & Sons, New York, 
1911.) Consequently whenever the organisms proceed toward or from 
a point thus located, it may be concluded that the stimulating effect 
of the light in the beams is equal, no matter how the light may differ, 
either in quantity or in quahty. (This is literally true for only a few 
organisms, but the principle as appHed holds for all, as will be demon- 
strated in the extended paper to follow.) It is, therefore, obvious that 
if the Ught in one beam is kept constant in quality, white for example, 
while that in the other is changed in color the relative stimulating 
