276 BIOLOGICAL EFFECTS OF RADIATION 



for reflection, in order to determine the amount of energy absorbed. In 

 case the reacting system absorbs all the radiation which enters the cell, 

 the measurement of transmitted light may be omitted, but usually it 

 is preferable to arrange the conditions so that some light is transmitted. 



Light Sources. — The purest monochromatic light is usually obtained 

 with a monochromator (or preferably a double monochromator) employ- 

 ing a prism which refracts the light into its different wave-lengths. Light 

 sources which give discontinuous spectra lead to a greater purity of 

 monochromatic light and the greater the distance separating the spectral 

 lines, the easier is the separation. The mercury arc is very convenient 

 and particularly suitable for photochemical investigation. Other metal 

 arcs are also available although they are less convenient (9). 



Monochromators are more effective in giving monochromatic light 

 than most filters, but they are subjected to the handicap of low intensity. 

 Only a small portion of the light from the source enters the mono- 

 chromator slit and a large portion of this light is scattered and lost in 

 the monochromator. Accordingly, the intensity of the exit beam is 

 very low. A considerable amount of effort has been expended in improv- 

 ing monochromators to give greater intensities. The prisms and lenses 

 have been increased in size and the focal length of the lenses has been 

 shortened. Also the intensity of the light has been greatly increased by 

 using capillary lamps (5) which concentrate most of the light of the 

 lamp on the monochromator slit. 



The principle of focal isolation has been used in rendering light 

 monochromatic. Light of longer (or shorter) wave-length is stopped 

 by focusing the polychromatic light onto a screen provided with a suit- 

 able small hole. In the short ultra-violet this method may be superior 

 to the monochromator method. The relative efficiencies of the two 

 methods in both intensity and purity have been studied by Heidt and 

 Forbes (8). 



Satisfactory filters for different wave-lengths have been worked out 

 in the visible region and various solutions and several convenient glass 

 filters are now available (4, 11). The situation in the ultra-violet is less 

 satisfactory and new developments in this field would be welcome. The 

 subject of filters and monochromators has been discussed in detail by 

 Brackett on pages 149 to 170. 



Measurement of Intensities. ^ — The most satisfactory device for meas- 

 uring the intensity of light is the thermopile, which consists of junctions 

 of unlike metal on the back of thin metal receivers which are blackened. 

 For photochemical investigations the utmost sensitivity is not required 

 because a fairly high intensity of radiation is necessary to effect sufficient 

 photochemical change to measure by chemical analyses. 



* Only techniques of special application to photochemistry are suggested here. 



