MB. W. CEOOKES OIN BEPULSION EESULTING- EBOM RADIATION. 
365 
The screens, whilst diminishing the heat almost to nothing, also cut off a consider- 
able quantity of the light. 
197. A similar series of experiments was tried with the radiation from a candle. 
No alum, glass, or water screens were at first interposed, and the deflections were taken 
with the clear and opaque disulphide, alternately put in the path of the ray. The candle 
was 3 feet off. The mean of several experiments was as follows : — 
Opaque disulphide 28 degrees. 
Clear disulphide 130 „ 
This shows that much of the action of a candle is due to rays which pass through 
iodine, i. e. to the ultra-red rays. 
The candle was now brought 2 feet from the apparatus, and the alum and glass 
screens were interposed. On repeating the experiments the mean result was as 
follows : — 
Opaque disulphide 5 degrees. 
Clear disulphide 37 „ 
Eig. 16 . 
198. I have hitherto taken it for granted that a lampblacked surface is the most 
repelled by radiation, and that a white surface, such as that of freshly cut pith, is the 
least repelled. Experiments tried repeatedly with other surfaces abundantly confirm 
this supposition. It was necessary, however, to get accurate data on this point ; and I 
have accordingly fitted up an apparatus which will enable me to measure the force of 
radiation and its action on disks of various materials of the same size, compared with a 
standard black disk. 
The apparatus is represented in fig. 16. It is similar in principle 
to the torsion-apparatus already described (186). a b is the hori- 
zontal tube containing the glass torsion-beam ; c d is the suspending 
fibre, also of glass. In the centre of the beam is a mirror, from 
which an index ray of light is reflected. The end a of the horizontal 
tube is sealed on to a wider piece of tube in a vertical position, and 
having in front of it a large opening {e f) occupying the whole of 
one side of the piece of tube ; this opening has the edges ground 
perfectly flat, and is closed with a piece of plate glass cemented on. 
The object of the opening is to enable the disks to be changed. At 
the end (b) of the horizontal tube is another opening, closed in like 
manner with a plate of glass. This is to give access to the pan ( g ) 
of the beam, so as to counterpoise the disks, as they are not all of 
the same weight. At the other end of the beam a light aluminium 
bar hangs, on which is cemented the standard disk h and the 
movable disk i. The standard disk is of pith, coated with lamp- 
MDCCCLXXVI. 3 E 
