MR. W. CROOKES ON REPULSION RESULTING FROM RADIATION. 
123 
however, the heat is communicated to the fixed driving wheel, and the fly then 
rotates positively. 
When plunged into hot water the fly revolves negatively, and then stops as soon 
as the temperature is equalised. On removing it from the water positive rotation 
sets up, and continues with considerable speed till quite cold. 
454. It was suggested by Professor Stokes that a disk might be made to revolve 
on its axis, and the following instrument was made in accordance with the design 
proposed. The disk is horizontal, mounted like the fly of a radiometer, and for 
liolitness sake is of mica, blacked above. Fixed to the bulb above the disk are 
four flat pieces of clear mica ; each extends from the side of the bulb to near the 
centre and ends below in a straight horizontal edge, leaving just 
space enough for the disk to revolve without risk of scraping. The 
edge is in a radial direction, and the plane of the plate inclined 
about 45° to the horizon, in the same direction for them all. 
Exposed to the light of a candle the rotation is against the 
edge, the same as in the instruments last described. 
It was found on experimenting that a much more sensitive 
instrument could be made by slightly modifying this form. Fig. 
26 shows the best construction of otheoscope with rotating disk. 
a a are six vanes of copper foil, oxidised by heating to redness in 
the am ; they are attached to arms, and are inclined at an angle 
of 45° to the horizon, as in the forms of driving vanes previously 
described. These are fixed to the support. Through the centre 
passes a needle point balancing a glass cup ; this carries a thin 
clear disk of mica, b b. freely rotating about a millimetre above 
the top edges of the copper vanes. When exposed to light, the 
mica disk rotates with great speed against the edges. If the 
lower part of the envelope is heated so that the pressure from 
the warm glass strikes against the vanes before reaching the disk, the direction of 
rotation is negative owing to the deflection of the pressure on passing between the 
vanes. As soon as the copper vanes have become warmed, they become the driving 
surfaces, and the rotation changes to positive. 
The upper part of the mica disk in one of these otheoscopes was divided into 
sectors, each of which was painted with one of the component colours of white light. 
When exposed to sunshine the speed was so great as to cause the colours to blend 
together into a neutral grey. 
The pressure which drives the movable fly round reacts equally on the driving 
surface (442). By suspending the driving vanes as well as the fly on needle points, 
so as to allow both to revolve freely and independently (345, 360), they both rotate 
under the influence of light in opposite directions. Otheoscopes have been con¬ 
structed of the forms shown in figs. 25 and 26, in which the upper and lower vanes 
li 2 
Fig. 26. 
