RESEARCH METHODS IX STUD} OE FOREST ENVIRONMENT. 49 



correlation of such measurements with observations on the condition, 

 rate of growth, etc., of the trees existing under the measured condi- 

 tions. The great advantages of this method are that a great variety 

 of light conditions may be obtained and maintained with little ex- 

 pense or trouble and that growth and health of the subjects may be 

 studied through long periods and under natural conditions. One 

 disadvantage is that a variety of light conditions is uecessaril) accom- 

 panied by a variety in the measure of other conditions the effect of 

 which may be confused with the effects of light, and aeither of the 

 two sets of effects can be exactly measured and balanced againsl each 

 other, nor, most of all, can they be controlled. A further disadvan- 

 tage consists in variation of the amount of shading at any given point 

 with different hours of the day and seasons of the year, necessitating 

 long-continued observations to obtain any expressive results. Thi 

 disadvantages, however, will loom up less formidably when we under- 

 stand better what part of the radiant energy is really effectn As 

 has been pointed out, the field method must go hand in hand with 

 laboratory studies. 



Apparatus and Methods for Radiant Energy Measurements 



Although most of the methods of light measurement used by forest 

 investigators have been described as now obsolete, it is impossible, of 

 course, to throw away all that has been gained through experiem 

 with different types of instruments. Quite apart from forest inves- 

 tigations, there is available a vast amount of researeh in the study 

 of light per se which, however incomplete and changing thi- study 

 may be, represents the starting point for any new work undertaken. 

 It is therefore considered expedient to bring together a list of the 

 methods and instruments which have been used, not in any degr< 

 of historical completeness, but rather to show the several line- of 

 study and their possibilities as briefly as possible. 



1. In the radiometer, which is commonly seen in jewelers' \\ Lndows, 

 the energy of light is transformed into work. This instrument has 

 no practical value, however, because the work is perforin,.! inef- 

 ficiently and probably does not vary in proportion to the energy 



received. 



2. The thermopyle represents the first attempt to transform radi- 

 ant enersv into electrical current. This is accomplished by allowim. 

 the light 'to fall upon the junction of two wires of differed metalf 

 The opposing ends of the two wires are also joined, forming a com- 

 plete circuit. The amount of current generated a. ssing around 



this circuit is measured at any point in the circuit by means ol a 

 galvanometer. The radiomicrometer, for measuring the heat iron 

 stars, is an extremely delicate adaptation of the principle o( the 



thermopyle. 



1 ( »i rq_92— Bull. 1 059 ! 



