RESEARCH METHODS I1ST STUDY OF FOREST ENVIRONMENT. 55 



Weisner (76) used an instrument almost as simple as this, his 

 standard shade and fresh paper being set in a groove in a block of 

 wood, and appearing in openings of a layer of opaque paper. 



Clements (6) devised a photometer in which a narrow strip of 

 ;; solio " or other sensitive paper may be held, having sufficient area 

 for 25 exposures. This strip is placed on the periphery of one metal- 

 lic ring, which fits snugly inside another. In the outer ring there is 

 an opening one-fourth of an inch square, covered by a slide which 

 is drawn back to make the exposure. In using this instrument the 

 colors obtained on exposure are not directly compared with a stand- 

 ard color. Rather, it is customary to make a scale of shades with 

 each set of observations, consisting of, say, 10 exposures in full light, 

 of 1. 2, 3, etc., seconds. In the later exposures, then, it is only neces- 

 sary to keep within the limits of the " scale," and the time may be 

 varied to secure the desired shade. If, for example, a 24-second ex- 

 posure gives a shade corresponding to 7 seconds on the scale for full 

 light, the relative value of the suppressed light is 7/24 or 29 per cent. 



The photochemical-photometer method is not satisfactory for any 

 expression of the light in absolute terms, or for comparing quantities 

 in one day or season with another day or season, or for comparing 

 different localities. Of course, all exposures might be compared to 

 some standard shade, but the operation is needlessly circuitous 

 and is made the more difficult by the perishable nature of the record, 

 the need for examining it in dim lamplight, etc. It is therefore be- 

 lieved that, while this method has some value, a similar effort ex- 

 pended in determining absolute light quantities, will be much more 

 profitable. 



In addition to the above there are instruments of the same prin- 

 ciple by which more or less continuous records may be secured. In 

 one such instrument, used by the Weather Bureau a number of years 

 ago, the light entering through a very small opening made its im- 

 pressiou as a band across the sheet of photographic paper on which it 

 fell at successive moments. (See Clements (6), p. 51.) 



3. It is probable that several hundred kinds of comparison pho- 

 tometers have been devised, all depending on an ocular comparison 

 of the sunlight to be studied, with a light of known luminous powers. 

 1 1 :- evident that such instruments deal only with the luminous rays, 

 ami while they rely upon the accuracy of the eye, the method cer- 

 tainly has advantages over the photochemical method in dealing with 

 thai pari of the spectrum which is least affected by ch a ages in atmos- 

 pheric absorption, rt is not to be supposed, however, that the lumi- 

 nous raj - control plant ad i\ it ies. 



One of the simples! types of comparison photometers is (he 



>ked glass type used by Wagner and credited to him by Zon and 



(7s). although undoubtedly invented much before his time. 



