SUNLIGHT AND ITS MEASUREMENT 191 



the incident radiation completely into heat and measuring the 

 heat thus obtained either directly in heat units (absolute units) 

 or indirectly in terms of its effects (arbitrary or relative units) 

 which may, after the necessary calibration of instruments and 

 computation of results, be re-stated in heat units. There is 

 no actual line separating heat and radiation of shorter wave 

 lengths so that thermometry is logically a branch of radiom- 

 etry. Likewise the very short waves, those termed in the 

 old text-books "chemical" or "actinic," 37 and the ultra-violet 

 rays are convertible into heat, so that if the radiometric instru- 

 ment is sufficiently sensitive (or the radiation is sufficiently 

 intense) the energy in these high frequencies also may be meas- 

 ured with it. 



Because the principle upon which radiometry is based is 

 valid for radiations of all wave lengths, this general method 

 has a greater range of applicability than either of the other 

 two. However, for the reason that it is usually applied 

 to investigations demanding great precision the instruments 

 are often delicate and complicated but this need not be so for 

 work where a very high degree of accuracy is not required. 

 These instruments all consist essentially of a receiver for com- 

 pletely converting the incident radiation into heat and a 

 device for indicating the amount of heat or of its effects. 



All receivers must depend upon a surface for the initial ab- 

 sorption, and all surfaces are subject to losses from reflections, 

 radiation and convection. A surface that is, to the eye, ab- 

 solutely black of course loses almost no visible radiation, but 

 it should be noted that some surfaces that appear black in 

 diffuse light may reflect a beam of light quite completely, es- 

 pecially when the angle of incidence approaches that of total 

 reflection. Besides providing that the instrument selected 

 shall be an adequate absorber of energy it is necessary to see 



37 One Li astonished at the persistent adherence to the conception of a dis- 

 tinction between "chemically active" and "non-chernically active" radiation 

 on the basis of wave length that is shown in modern botanical literature, es- 

 pecially when one recalls that even Sachs (Sachs, Julius, Vorlesungen iiber Pflan- 

 zen-Physiologie, p. 367. Leipzig. 1882.) insisted upon its erroneous character. 



