SENSIBILITY OF VOLVOX TO SPECTRAL LIGHTS 347 



lengths represents 'color' to a non-differentiated bit of proto- 

 plasm, is to confuse physical, physiological, and psychological 

 qualities of radiation. 



The plan of work outlined includes the carrying out of experi- 

 ments with lights such as we have described, to determine the 

 visibility of light in the different parts of the spectrum, as well 

 as the relative effects on the size of the pupil. Furthermore, the 

 study of the action currents of the eye of various animals when 

 stimulated by the various wave-lengths will furnish data and 

 information regarding the relations between the photochemical 

 and photo-electric effects. Determinations of the relative 

 effectiveness of the same or similar lights for different organisms 

 will furnish a fundamental basis for comparison with the relative 

 stimulating value, as determined by luminosity curves, for the 

 human eye. 



APPARATUS 



The apparatus used to obtain the lights of different wave- 

 lengths but of equal radiant power is described in the first paper of 

 the series (Laurens and Hooker, '17). In addition to the 

 twenty-three lights there listed, each 30 mix wide, and extending 

 from X 420 m// to X 670 m/z, a white light was made equal in ra- 

 diant energy content to the various spectral lights. This was 

 used in a balanced relation to the various spectral lights, as 

 will be described below. 



A small glass aquarium (26 mm. x 26 mm. and 10 mm. deep) 

 was made to hold the organisms under observation. For initial 

 orientation in the beam of light, the organisms were placed in 

 a trough made by placing two strips of celluloid parallel to 

 the direction of the beam of light. The aquarium was placed 

 on the stage of an ordinary dissecting microscope, with a glass 

 plate in the stage aperture. Twelve centimeters below the level 

 of the stage there was a ruby glow-lamp on a weak Columbia 

 dry cell, giving a very faint illumination. This lamp was 

 needed only when the shorter or longer wave-lengchs, or the 

 white light were being used, the organisms being clearly visible 

 in the majority of the spectral lights. It had no demonstrable 



