VISIBLE AND NEAR-VISIBLE RADIATION 129 



If the relative effectiveness for the standard source is fixed for each wave- 

 length, the only necessary demand is that the standard source be con- 

 stant and reproducible. Then other sources can be evaluated for each 

 wave-length in terms of the standard source, and the total contribution 

 obtained by addition. As a matter of practical procedure, however, 

 appeal is made to the absolute system of radiation measurements and the 

 standard observer defined as to his relative response to equal energy at 

 each wave-length. At once, when the response of the standard observer 

 is defined for each particular wave-length for a given energy, the photo- 

 metric system presents a known relationship to absolute radiation meas- 

 urements. Thus, so long as radiation occurs in the range of visibility, 

 conversion may be made from one system to another, though such con- 

 version may be tedious and inconvenient. Unfortvmately, however, 

 common procedure is made dependent upon the characteristics of the 

 particular observer. Consequently, discrepancies may arise through 

 his departure from the standard observer. 



In pure research, one might prefer to avoid discussion of spectro- 

 photometry. Practically, this is not possible for two reasons : (a) Radio- 

 metric apparatus is most commonly specified in spectrophotometric 

 units. (6) Spectrophotometric methods may be the most readily 

 available. 



Colorimetry, the third system, deals not only with intensity as it 

 affects the eye, but also with wave-lengths and combinations of wave- 

 lengths as they affect the eye. Thus, a multiplicity of combinations of 

 wave-lengths and intensities may produce an identical effect, so far as 

 the observer is concerned. Their color characteristics are therefore 

 identical. Thus, the system of colorimetry deals with radiation wholly 

 from the standpoint of the visual observer. It is concerned with the 

 objective characteristics of radiation only so far as it is necessary in order 

 to understand the subjective response. No detailed discussion of this 

 system will be undertaken. 



In the first chapter, terms which have arisen in connection with these 

 three systems have been used more or less indiscriminately, the assump- 

 tion being that at all times our attention has been focused on the purely 

 objective phenomena and that measurements would be made in a purely 

 physical system. If we could avoid a discussion of photometry and 

 colorimetry, no further distinctions would be necessary. Since, however, 

 in practical procedure, it is not possible to follow this simple expedient, 

 it will be necessary to distinguish sharply between the concepts of the 

 different systems. In making such a distinction, it is further necessary 

 that ambiguity in the use of the terms be avoided. 



Thus we shall draw a sharp distinction between the purely physical 

 phenomenon of radiation, the objective stimulus of vision, i.e., light, 



