364 
ME. W. CEOOKES ON EEPTTLSION RESULTING FEOM EADIATION. 
of iodine in disulphide of carbon. This was opaque to the sun’s ray. On opening the 
shutter the deflection amounted to 79°. 
This experiment shows that the total radiation from the sun, passing through alum 
and glass screens, produced a deflection of 300°, whilst if the whole of the light was 
cut off by interposing a solution of iodine in disulphide of carbon, there was still a 
movement of 79°. This 79° was the effect due to dark heat which penetrated the alum, 
the difference (221°) being due to light. 
I endeavoured to cut off the whole of the dark heat, so as to work with the lumi- 
nous portion only of the solar radiation. The ray of sunlight reflected from the 
heliostat was accordingly passed through the following screens, placed one behind the 
other, as shown in Plate 36. fig. 15 : — 
A lens. 
A thick, total-reflection, right-angled prism. 
Three thin plates of glass. 
A plate of alum 7^ millims. thick. 
Two thick glass plates. 
Another plate of alum, 5 millims. thick. 
A glass cell full of saturated solution of alum. 
An empty glass cell. 
It was expected that these numerous cells would effectually cut off the dark heat- 
rays. To prove this the empty cell was filled with opaque disulphide*. On opening 
the shutter the deflection of the index was 2° only. 
What came through the alum, glass, and water screens was therefore pure light, 
practically free from the dark rays which are called heat. 
The opaque disulphide was then replaced by clear disulphide. The deflection was 105°. 
Action of 
filtered sunlight. 
Opaque disulphide 2 degrees. 
Clear disulphide 105 „ 
Deducting the 2° as representing the trace of dark heat which escaped the alum, 
glass, and water screens, the difference (103°) represents the action of the luminous 
portion of solar radiation and of that small quantity of the ultra-violet rays which 
would pass the screens ; for had the effect of the sun’s radiation after passing through 
the screens been due to heat , on cutting off the light by means of opaque disulphide, 
the deflection should have been practically undiminished. But experiment shows that, 
after passing through the screens, the repulsion due to heat is less than 2 per cent, of 
the total action of the solar ray. 
* Eor the sake of brevity I call the solution of iodine in disulphide of carbon opaque disulphide : the disul- 
phide of carbon alone I call clear disulphide. 
