VISIBLE AND NEAR-VISIBLE RADIATION 197 



combination of photocell and short-wave cut-off filter, a band of sensi- 

 tivity can be secured anywhere in the visible or ultra-violet. It should 

 be pointed out, however, that the materials indicated in Fig. 32, (2) are 

 markedly superior in reproducibility of sensitivity to any of the other 

 materials. 



Through refinement of manufacture, the commercial cells, however, 

 have been brought to a high degree of perfection. Even so, the experi- 

 menter must be cautioned in the use of photocells. No two photocells 

 are alike, nor is any photocell permanent. The sensitivity curve of the 

 particular cell must be determined at the time of use by means of a 

 standard source. For an exhaustive discussion of photocell technique, 

 the reader is referred to Hughes and Du Bridge (39). 



PHOTOCHEMICAL METHODS 



Another general type of selective detector is the photographic or 

 photochemical method. Such a method derives its wave-length sensi- 

 tivity from the photochemical sensitivity of the reaction. Thus, the 

 darkening of the photographic plate may serve as an indicator for the 

 range of sensitivity of the plate. Photographic plates are of course 

 readily prepared with a wide range of sensitivity distribution. (See 

 Eastman Kodak Co. literature.) These in turn may be combined with 

 various filters in order to secure modified ranges of selective response. 

 Such methods, however, are nonlinear as to intensity and depend upon 

 the time of exposure. They are, therefore, nonlinear indicators of time X 

 intensity dosages. It is difficult to bring them in line with other quanti- 

 tative methods. They require the difficult operation of evaluating 

 darkening. However, because of their convenience and simplicity, they 

 may be recommended for restricted types of observation. These methods 

 have been highly developed for spectroscopic purposes and are, when 

 properly used, capable of high precision. For a comprehensive discussion 

 of this field, the reader is referred to the works of Harrison (37) and of 

 O'Brien (55). 



Another photochemical method deserves particular mention. When 

 lithopone or better, pure zinc sulphide, moistened with lead acetate, 

 is exposed to the ultra-violet, a marked darkening takes place. In order 

 to make the results of this method reproducible, only the purest chemicals 

 must be used. The darkening must be brought to a certain specified 

 standard value and the time required accurately determined. This 

 method has been extensively applied to solar observations by Clark. 

 In order to give an idea of the sensitivity distribution, values obtained by 

 Brickwedde (13) are plotted in the middle section of Fig. 30, marked 

 ZnS-1. In order to further restrict the long-wave-length cut-off, Clark 

 (18) developed a differential method in which the time of darkening is 



