98 Thh Plant World. 



course the angle at which the instrument is to be placed may be 

 determined beforehand by reference to the almanac. It has 

 been found convenient to mount the cups on a flexible tube of 

 lead or copper, though a rubber section inserted between two 

 glass tubes is as satisfactory, providing a proper support for 

 the cup be arranged. 



To blacken the cups, ordinary lamp-black is clarified by 

 boiling in distilled water, cooling and decanting, and repeating 

 the process several times. The carbon paste thus obtained may 

 be diluted with distilled water and applied to the cup (after the 

 latter is filled and placed in operation) by means of a small 

 brush. A little practice renders the operation easy. 



The blackened cup often, though not always, shows a 

 somewhat greater sensitiveness to sunlight than the brown one. 

 Whether this depends upon the quality of the light or on some 

 other factor has not been investigated as yet. To have the re- 

 sults comparable, the same form of cup must of course be used 

 throughout any series of observations. 



The dark cups function as does the white one, to integrate 

 the efifects of the evaporating power of the air, and they add to 

 these the efi"ects of their more complete absorption of radiant 

 energv. A dark cup in darkness or very weak light operates 

 just as does a white one, while in strong light, especially in full 

 sunshine, the difi"erence between the readings is very marked. 



Since the radio-atmometer, as so far constructed, sums the 

 effects of absorbed radiant energy only in so far as these effects 

 are in the direction of heating the surface of the instrument 

 (that is, only in a photo-thermal way), the readings obtained 

 therewith bear no necessarily direct relation to the light condi- 

 tions as these determine the photosynthetic process of plants, 

 which is photo-chemical in its nature. According to the re- 

 searches of Blackman and Matthaei, * however, we may con- 

 sider that temperature is more important for photosynthesis 

 than is light intensity, providing of course that there is a con- 

 siderable amount of light, and our instrument takes account of 

 temperature as this would influence plant leaves. At any rate, 

 we are able to compare the varying intensities of sunlight for 



♦Blackman, F. F. and Matthaei, G. L. C, Phil. Trans. Roy. Soc. I,onci. Ser. B, Vol. 197. 1904 



