178 NATURE STUDY REVIEW [9:6— Sept., 1913 



2. Purpose. — To determine optimiun light. ^ 



Materials. — Two granite iron bread pans, black paper and 

 different thicknesses of other paper. 



Procedure. — Place an equal number of animals in two bread 

 pans. Cover one entirely with black paper. Over the other 

 arraijge a graded series of papers having at least four divisions; 

 one with one thickness of tissue paper, the next with one thickness 

 of thin writing paper, the next with heavy writing paper, and the 

 foiirth with black paper. Put both pans under a tungsten lamp 

 placed 2o inches directly above the animals. Arrange a heat 

 screen as outlined in Expt. i. If necessary to obtain definite 

 response add to, or subtract from the different thicknesses of 

 paper. It will be necessary to stir the animals so as to redistribute 

 them just before each rearrangement of the paper. Observe and 

 record results as before. 



Results. — The conditions of this experiment give a series of 

 varying light intensities. The light in which the animals come to 

 rest will be the optimum light for them. Isopods and amphipods 

 will collect at the dark end while the common and fairy shrimps 

 will be under the thinner paper. 



Discussion. — This experiment may be run in the diffuse light 

 of the room or with any light-giving device that does not cause a 

 shadow to be cast by the sides of the pan. With most animals 

 all the comers of the pan should be rounded out with wax to 

 prevent thigmotactic response. This experiment gives quite 

 definite results when tried with isopods, amphipods, or pond 

 snails (negative to light) . 



3. Purpose. — To determine the reaction to colored light. 

 Materials. — Two bread pans and three colored glass plates 



of equal thickness, one red, one green, one blue. Colored tissue 

 paper can be used but the results are less definite. 



Procedure. — Using the light described above replace the graded 

 series of paper by the three colored glass plates. Test the reactions 

 of the animals to all possible combinations of the colored plates 

 and find which color is least stimulative. That is, under which 

 color will the majority of the animals come to rest ? 



^By optimum light is means the light condition under which the activities 

 of the animal are carried on at their most favorable rate. This would there- 

 fore be the light condition which would least stimulate the animal under 

 experimentation . 



