CoNGDON, Reactions to Light. 313 



The first experiment definitely directed to determining the relation of phototaxis 

 and the image-forming power of the eye is described by Parker ('03a). He made 

 use of the positively phototactic butterfly, Vanessa. The animal was placed in such 

 a position between a window and a candle that the intensities of light from the two 

 sources were equal where they fell upon the animal's body. Under these conditions 

 Vanessa flew towards the window, thus demonstrating that it can distinguish 

 between the size of luminous fields. Phototaxis is preceded by a choice of the field 

 to which it orients. The experiment, as Parker points out, furnishes an answer 

 to the query why positively phototactic winged insects do not fly towards the sun. 

 They seek instead the larger mildly illuminated patches upon the earth's surface. 



Cole ('07) employed Parker's test upon a number of terrestrial animals and 

 thus increased our knowledge as to the relation of phototaxis and the power of 

 forming primitive images. The animals were placed perpendicularly to the line 

 joining two parallel screens and equidistant from them. The light given off by the 

 screens per unit surface was inversely proportional to their size. Therefore the 

 total light intensities of their surfaces were equal. The dung worm AUolobophora, the 

 insect larva Tenebrio, the cockroach Periplaneta, the European garden snail Helix, 

 and the blinded cricket frog Acris did not give a greater number 0/ turnings to one 

 field than to the other. On the other hand the flatworm Bipalium, and the small 

 crustacean Oniscus showed some little power of discrimination. Vanessa, Ranatra, 

 and two species of frogs with eyes intact distinguished readily between the screens 

 and always oriented to a particular one of them. 



A discussion of the relation between perception of detailed images and photo- 

 taxis appears in a recent work upon vision by Nuel ('04). 



trial. 



Jennings was the first to apply the idea of trial, long recognized for vertebrates, 

 to invertebrates as well. We shall consider the papers on this subject relating to 

 the earthworm by Parker and Arkin, Smith, Adams, Holmes and Harper before 

 turning to the protozoan studies of Jennings. 



The methods used by Smith ('02) in studying the earthworm are valuable in 

 giving, as it were, a birdseye view of its activities in light of rather w^eak intensity. 

 She devised a means of plotting upon paper the path of the worm for a considerable 

 distance. Exploring movements were shown by spurs upon the line indicating the 

 animal's course. When worms are started with their bodies perpendicular to hori- 

 zontal light, they go in various directions, varying from directly toward to directly 

 away from the light. In the great majority of cases the course is obliquely from the 

 light. Exploring movements are especially common when the anterior end of the 

 worm encounters stronger illumination or an unfavorable surface. Often they are 

 preceded by a recoil. Although the fact is not emphasized by the writer, her 

 diagrams show that exploring movements toward the light are not followed up so 

 frequently as those away from it. 



We must turn to Holmes ('05) for the application of the trial idea to the worm. 

 He makes the statement that the first effect of moderate light upon the earthworm 

 is the production of exploring movements, of the anterior end, haphazard as to 

 direction, with possibly a few more away from the light than toward it. The sec- 

 ond effect is to check the movements toward the light. As a result the animal 



