RESEARCH METHODS IN STUDY OF 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 necessarily accom- 

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

 which may be confused with the effects of light, and neither of the 

 two sets of effects can be exactly measured and balanced against 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. These 

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

 stand better what part of the radiant energy is really effective. As 

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

 laboratory studies. 



Apparatus and Methods foe 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 experience 

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

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

 of light per se which, however incomplete and changing this 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 degree 

 of historical completeness, but rather to show the several lines of 

 study and their possibilities as briefly as possible. 



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

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

 no practical value, however, because the work is performed inef- 

 ficiently and probably does not vary in proportion to the energy 

 received. 



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

 ant energy into electrical current. This is accomplished by allowing 

 the light to fall upon the junction of two wires of different metals. 

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

 plete circuit. The amount of current generated and passing around 

 this; circuit is nu'asiin;d at any point in the circuit by means of a 

 galvanometer. The radiomicrometer, for measuring the heat from 

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

 fchermopyle. 



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