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parallel sides was set and was supported by a shoulder around tlie in- 

 side of the box as shown in the figure. The glass container was partly 

 filled with water mixed with a little Higgen's Waterproof Ink, (c) so 

 that when the container was in position there was formed a prism of the 

 solution, thick at one end and thin at the other. The surface of the 

 liquid made an angle of 5° with the bottom of the container. 



Direct sunlight was used in the experiments since it was the only 

 kind of light obtainable that would answer. In the bottom of the box 

 was placed a rectangular glass dish, [d) 36 cm long, 23,5 cm wide and 

 7,5 cm high almost filled with fresh sea water and lined with dead black 

 paper. 



By placing the apparatus so that the sun's rays entered the prism, 

 as shown in the figure (e), it was possible by means of the layer of water 



and Higgen's ink to obtain a lighted area in the dish of water which 

 was of graded intensity. 



The method employed in using the above apparatus was as follows: 

 Echinasters of various ages were used and these were introduced into 

 the dish, care being taken to vary the position of the rays , and to vary 

 the manner of handling, so that these factors should not influence the 

 direction of movement. 



Several series of experiments were undertaken, some with the sun's 

 rays directed toward the thin end of the prism, some with the rays 

 directed toward the thick end and others with the rays practically 

 vertical. In every trial the starfish moved apparently without hesitation 

 to the brightest part of the graded field, except in the case of one 

 individual the tips of whose rays had been amputated several days 

 previous, and even this individual moved slowly into the brightest 

 region. In all cases the starfish stayed at the lightest end some little 

 time. 



