THE ORIENTATION OF PORCELLIO TO LIGHT 117 



exposing sensitive individuals suddenly to lateral illumination. 

 The individual to be observed was placed on a smooth dead 

 black ground, in a dark room. When its orientation had been 

 accurately determined by means of a 25 or 60 w. tungsten bulb 

 a few inches behind it, away from which it was moving, or a 

 distant light in the ceiling, another tungsten bulb of either 25 

 or 60 w. and at different distances varying between 20 and 

 40 cm., was suddenly turned on, so that its light should strike 

 the animal from the side at an angle as near ninety degrees as 

 possible. Sometimes at the instant the lateral light was turned 

 on, all other lights were extinguished; at other times, they were 

 not. In both cases, the direction, with reference to the lateral 

 light, of the first movement of the organism out of its course 

 was determined. 



These experiments, simple as they were, gave results that were 

 strikingly definite and convincing. Almost invariably- the first 

 movement was away from the lateral light. The reaction was 

 sharper, on the whole, when light came from the side only. 

 To the 60 w. light, at 40 cm., the response was more definite 

 than to the 25 w. light at the same distance. But the reaction 

 was unmistakably negative within the limits of variation of 

 lighting and distance mentioned. A significant feature of the 

 results was the ease with which they were obtained and the 

 simplicity of means employed. 



It must be remembered that all individuals are not equally 

 sensitive to light. But the consistency with which many indi- 

 viduals turned away from. the light, whether the latter was on 

 one side or the other, left no room for doubt that the reaction 

 was forced in a definite direction. 



9 



The second method of experimentation, equally simple, was 

 determined by the fact that many individuals responded more 

 readily to light coming from behind than from in front of them. 

 The following series of observations taken one afternoon are not 

 selected, but indicate the reactions of the first individuals tested. 



The lamps used in these experiments gave a source of light 

 4 to 5 cm. in diameter. This fact it is important to keep in mind 

 when considering the definiteness of the responses of Porcellio 

 for the smaller angles of incidence recorded in the tables. For 



