122 
MB. W. CROOKES OK REPULSION RESULTING FROM RADIATION. 
with considerable speed. The black disk is here the driving surface, the glass envelope 
not being active. 
A more sensitive form of this construction is made by having a fixed copper disk 
lampblacked on the upper side, and making the vanes of mica for the sake of light¬ 
ness, fig. 25, B. The vanes are as numerous as can be conveniently put together, 
and being set at an angle, the pressure from the copper plate drives them round 
with great speed when set in action with even a faint light. Another form con¬ 
sists in suspending on the fly aluminium cups, turned at an angle of 45°, with the 
concave side downwards, as shown in fig. 18, No. 4 (432). This form likewise 
proyed very sensitive. 
453. The pressure being exerted chiefly at right angles to the driving surface, 
it was thought that power was lost by the tangential action of the pressure in the 
above form of otheoscope. Other instruments were therefore made with the driving 
surface of thick copper, cut into 16 equal portions by radial lines stopping short 
of the centre. Each of the 16 sectors is then bent round to form an angle of 45° 
with the original plane of the disk. This driving wheel is lampblacked and fixed 
to the support in the centre of the bulb. Immediately above, and suspended on 
a needle point, is another disk cut into sectors, and bent at an angle exactly the same 
as the lower disk. As the upper and lower sectors face each other, being in parallel 
planes, a force acting at right angles to the driving surface will also strike the movable 
fly at right angles, and drive it round in a more direct manner. Experience shows 
that this form is attended with advantage. 
These otheoscopes are very sensitive to heat externally applied. If those shown 
in fig. 25 be warmed at the upper part above the movable fly, the fly is driven round 
in the negative direction ; the pressure which acts on the vanes from above giving 
the opposite rotation to that produced when pressure acts from below. 
If the lower part of the case is warmed, no action is seen at first; gradually, 
