54 BULLETIN 1059, U. S. DEPARTMENT OF AGRICULTURE. 



There is hardly any question that an instrument of this kind may 

 be made to serve the present practical requirements of forest studies 

 for a measure of radiation intensity. It would be very desirable to 

 have the necessary protection from air currents supplied, without 

 intercepting some of the rays by a layer of glass. 



7. Photochemical photometers .—The objections which have been 

 raised to the use of chemical reactions as a measure of the sunlight 

 intensity can not be overcome. In addition to the fact that the other 

 rays may not vary from time to time in the same proportion as the 

 chemically active rays measured, it is somewhat questionable whether 

 the result secured, as in the coloration of a photographic paper, is 

 proportionate to the product of the light intensity and the time. 



In forest studies the photochemical method seems to serve one 

 purpose fairly well, that being to obtain a measure of the density of 

 the canopy. It is then assumed that the amount of light reaching 

 the ground is to the total sunlight as the area of openings in the 

 crowns is to the whole area; or, in other words, that the lighi coming 

 through these openings is unaltered in its passage. While technically 

 there is also light below the crowns, which has been transmitted 

 through the leaves, and the photographic paper may be sensitive to 

 the rays in this class, it probably does not introduce any serious 

 error, considering the purposes for which such measurements should 

 be used. 



This crown-density determination should always be made by mov- 

 ing the photometer through as great a space as possible during the 

 few seconds of exposure. 



The Bunsen-Roscoe (55) unit of actinic light is the light required 

 to produce on a standard paper a shade equivalent to that produced 

 by the mixture of 1 part lampblack and 1,000 parts pure white zinc 

 oxide. The details of the preparation of this "normal shade' and 

 the " normal paper" are given by Zon and Graves (78) or may be 

 obtained from the original citation given above. The object in men- 

 tioning it here is simply to show that it is possible to carry on photo- 

 metric observations on a fixed standard. 



Likewise, photographic-supply manufacturers have prepared a 

 standard shade and a standard paper for estimating light intensi- 

 ties. One of the best known of the " exposure meters' 1 is extremely 

 simple in operation. In one opening of a small disk containing the 

 standard paper is exposed the standard color, a permanent shade. In 

 a corresponding opening may be seen a fresh area of the paper. It 

 is only necessary to expose this to the light until it attains the stand- 

 ard shade noting the time required, to have a very close basis for 

 computing the light intensity. It seems that this simple contrivance 

 may serve the purpose of ecologists quite as well as more elaborate 

 apparatus of the same type. 



