Intelligence and Miscellaneous Articles. 317 



Hence 



M-M'==/X*-/3); 



or if /?=0 (that is, the medium is adiathermanous for obscure rays), 

 M-M'=focs. 



I instituted the experiments both with the light of a Locatelli's 

 lamp and with that of the sun. But as the intensity of the rays in 

 the former case was so small that no perceptible phenomena of fluo- 

 rescence were produced, I laid no weight on the results obtained, 

 which were entirely negative, although I used a delicate reflecting 

 galvanometer ; and in my subsequent experiments I used sunlight 

 exclusively. But even here material difficulties are met with. The 

 alteration of the radiation, due to the varying heights of the sun 

 and to the changes in the condition of the atmosphere, must be care- 

 fully alio wed for if mistakes are to be avoided.. Hence those experiments 

 alone can yield useful results which are obtained at midday — that is, 

 when the height of the sun is constant, or nearly so, and with as 

 cloudless a sky as possible. The experiments were made by reflect- 

 ing the light brought horizontally into the chamber by a Silbermann's 

 heliostat on a second metallic mirror suitably inclined to the horizon. 

 The light reflected from the metallic mirror was collected in a quartz 

 lens of pretty long focus placed in a metal screen, and was then 

 thrown on the open surface of a fluorescent liquid. This latter was 

 contained in a shallow dish between two metal screens, which were 

 provided with apertures for the passage of the incident and reflected 

 rays, and fixed in a suitable manner. The thermopile was pro- 

 vided with a short additional tube on the side turned to the source 

 of heat, and was placed quite near the aperture of the posterior 

 screen, so that it could take up the reflected pencil of rays as com- 

 pletely as possible. 



For measuring the current, an ordinary astatic thermomultiplier 

 made with silver wire was used. A third screen was used to cut 

 off" or admit at pleasure the incident rays ; it was between the quartz 

 lens and the front perforated screen. I originally spread the fluo- 

 rescent liquid in a thin layer on a ground-glass plate, but subse- 

 quently preferred placing it in a watch-glass. Care was taken that 

 the height of the liquid should be such that the reflected pencil of rays 

 could completely reach the thermopile. For the interposed plate 

 I used an alum plate about 2 millims. in thickness ; and I tried, 

 besides this, a dark-violet glass which transmitted no red rays, as 

 well as glasses of other colours (green, blue, and brown). As 

 fluorescent liquids, I used an ether alcoholic extract of dry leaves 

 (solution of chlorophyll), and the splendid red fluorescing substance 

 prepared by Rochleder from sesculine (llochleder's solution of 

 sesculine). 



Denoting the two positions of the interposed plates as anterior and 

 posterior, according as they were placed in reflected or transmitted 

 light, the following results were obtained : — 



