REACTIONS OF AMEBA TO LIGHT. 4J 



the chances are about even that the ameba came into contact 

 with the subsidiary image first, and therefore the likelihood of a 

 positive response is increased. From my own experience I have 

 found that whenever mirrors are necessary, it is absolutely es- 

 sential that they are silvered on the front surface; otherwise sub- 

 sidiary images will result, and in case monochromatic spectral 

 light is used a considerable degree of impurity may occur. If 

 Mast used back surface mirrors in his work on the light reactions 

 of ameba, some of his experimental results are, therefore, in- 

 completely described and consequently inconclusive; and his 

 inability to observe positive responses was due, in part, to im- 

 proper staging of the experiment for this purpose. 



The great majority of the light experiments in this paper show 

 positive reactions; a few are indifferent, and a few negative. 

 This proportion resulted from the manner in which the experi- 

 ments were set, and from the fact that observations were based 

 upon the behavior of amebas before they came into contact with 

 the beam of light as seen by the eye, instead of after, as previous 

 observers did. But in a great many cases negative behavior did 

 not result even after the ameba came into contact with the light; 

 but if the beams had been larger it is not improbable that the 

 proportion of negative reactions would have been larger. 



These experiments on the reactions of ameba to light were 

 performed not only for the purpose of testing the sensitiveness of 

 these organisms to light by itself, but especially to see whether 

 differences in intensity, quality or direction of light rays are 

 capable of causing changes in the behavior of amebas while 

 feeding. In order to let the results speak as definitely as possible, 

 a number of experiments were performed first with beams of 

 light only as stimuli. Later, particles of food were presented in 

 connection with the beams of light. By comparing these two 

 sets of experiments with each other and with the results of pre- 

 vious experiments on the feeding habits of ameba (see Bibli- 

 ography for references) the effect of light on feeding may be 

 readily observed. 



Several investigators, as already pointed out, made observa- 

 tions on the reactions of ameba to light, but all of them made 

 use of large areas of very intense light. But for purposes of 



