450 Prof. C. Timiriazeff. [Apr. MO, 



This disposition being rather troublesome, as it implies the use of a 

 dark room and of such expensive and cumbersome apparatus as the 

 large heliostat and the optical bench, I devised later the following 

 more simple and portable form (Plate 22). This time it was the 

 law of the cosines that was applied to measure the intensity of the 

 incident rays. A small metallic platform movable on .a vertical axis. 



FIG. 9 



and at the same time easily adjustable in the direction of the sun- 

 beam, bears four smaller movable platforms with four glass tubes 

 containing leaves. The four platforms may be put at such angles that 

 the light received by the leaves will vary in the same ratio 1, ^, j, ,!, 

 (Plate 22, A). As the experiment lasts only 15 20 minutes, the 

 observer standing by may re-adjust three or four times the position <>i 

 the large platform, so as to keep the glass tube of the first small plat- 

 form at right angles with the falling sunbeam. The loss of light dm- 



