l82 CHAP. XXI. THE ENERGY OF THE SPECTRAL RAYS 



outside air, which, again, should not be disturbed in any 

 way. The observer never moves from his place in the dark 

 room, which is closed on all sides. 



A freshly mounted sensitive strip was found to be 

 erratic in its responses, the zero-point being continuously 

 displaced, as the result of the physical strain to which it had 

 been subjected by previous filing and in mounting it in the 

 apparatus. The instrument was therefore allowed to remain 

 undisturbed in the experimental room for a fortnight, after 

 which the responses became extraordinarily consistent ; 

 the zero-keeping quality also became very perfect. When 

 the sensitive strip was exposed to a particular radiation, 

 the maximum deflection was attained in the course of two 

 seconds ; stoppage of radiation was followed by an almost 

 immediate return of the spot of light to the exact zero in a 

 dead-beat manner. As regards the mica cover, a statement 

 is often made that mica is opaque to certain radiations. 

 This is true only when the thickness is sufficiently great ; 

 a thin strip put ' on ' and ' off ' the path of radiation did 

 not produce any change in the deflection of the Radiometer. 



The spectrum produced by the carbon disulphide prism 

 was found, as already stated, to extend beyond the limit of 

 the visible red, this extension into the infra-red region being 

 almost 6 cm., or about one-third the breadth of the visible 

 spectrum. 



The extreme sensitiveness of the Radiometer was also 

 exhibited by its discrimination of the radiation-components 

 of the morning and the midday light. In the morning the 

 intensity of the blue rays in the spectrum was found to be 

 slightly less than at midday. This is due to the greater 

 scattering of the short waves by the thicker stratum of the 

 atmosphere through which the light has to pass in the 



morning. 



The experiment on photosynthesis was carried out 

 between 10 and n a.m. on March 10. The energy of 

 radiation in different regions of the spectrum was measured 

 on January 16 and 19, 1923, and on March 10 immediately 



