PHYSICAL AND METEOROLOGICAL RESEARCHES, PRINCI- 

 PALLY ON SOLAR RAYS, MADE AT THE STATION 

 OF AGRICULTURAL CLIMATOLOGY AT 

 THE OBSERVATORY OF JUVISY. 



Camillk Flammarion, 

 Director of the Station. 



The purpose of the station of agricultural climatology is to study the 

 sun's rays and their action upon the phenomena of plant growth. The 

 observatory at Juvisy is constantly engaged in observing the sun and 

 its changing surface. It measures the size of the sun spots and tries 

 to fix the astronomical bases upon which rest the relations between the 

 variations in terrestrial temperature and the source of all the heat and 

 light of the earth. The station studies the absorption of heat and light 

 rays by plants, and analyzes the action of the different colors of the 

 solar spectrum. It observes also the different transformations of the 

 energy of the sun on which depends so intimately the entire terrestrial 

 life. 



SOLAR RAYS AND THE DEVELOPMENT OF PLANTS. 



Special attention has been given to the study of the peculiar action 

 of the sun upon vegetation. The relative efficiency of the light rays, 

 the heat rays, and the chemical rays has been studied. In this respect 

 the slow rays of the extreme red of the spectrum differ widely from 

 the rapid rays of the extreme violet. The rays which exercise the most 

 favorable influence upon certain phenomena of vegetable life are readily 

 investigated. For such investigations hothouses were built at the 

 station. The glass used was subjected to a careful spectroscopic exami- 

 nation. Blue glass, approaching closely to violet, was obtained, which 

 was only traversed by the rays of the extreme right of the spectrum. 

 The red glass was almost monochromatic and was only traversed by a 

 little orange. The green glass gave least satisfaction. The three hot- 

 houses were placed close together under the same meteorological con- 

 ditions. Adjoining the houses which were covered with colored glass 

 was an ordinary hothouse, for comparison, with full illumination. The 

 houses were ventilated by a current of air passing from south to north 

 through them, the object being to provide as nearly as possible natural 

 conditions and to avoid overheating. The temperature of the air in 

 the different hothouses was observed and the quantity of the sun's 

 heat which traversed them was measured. The accompanying table 



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