CHAPTER VII 

 ACTION OF LIGHT UPON PROTOPLASM 



IN this chapter it is proposed to discuss (I) the application 

 and measurement of light ; (II) its chemical action ; (III) the 

 effect of light upon the general functions of organisms; and 

 (IV) the control of locomotion by light phototaxis and 

 photopathy. 



1. THE APPLICATION AND MEASUREMENT OF LIGHT 



Light, which as a form of radiant energy is closely related to 

 radiant heat, is always accompanied by a certain quantity of 

 heat, whose action (in at least one control experiment in every 

 set) should be eliminated. To cut out heat without great loss 

 of light, we must employ transparent adiathermal media. Of 

 these, a plate of ice is the most effective, but alum, on account 

 of its higher melting point, is more convenient. A parallel- 

 sided vessel full of distilled water, or, still better, a saturated 

 aqueous solution of alum, forms an inexpensive, highly adi- 

 athermal screen. 



The quality of the light used in any experiment should be 

 carefully determined. If any other light than that of the sun 

 or incandescent solids is employed, it should be subjected to 

 spectroscopic analysis. For biological purposes a direct- vision 

 hand spectroscope, such as BROWNING'S, is convenient and 

 adequate. 



Often monochromatic light or a definite range of the spectrum 

 is desired. This may be obtained in various ways. The 

 purest monochromatic light can be got by making a long spec- 

 trum and using the desired part of it. To make such a spec- 

 trum one may employ, in a dark room, a lamp, followed in 

 succession by a slit and a lens to form an image of the slit 



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