MR. W. CROOKES ON REPULSION RESULTING FROM RADIATION. 
121 
the part corresponding with the cooler of the engine (the side of the glass bulb) admits 
of but little modification. It must almost necessarily be of glass—one of the worst 
materials for the purpose ; it is obliged to be of one particular shape ; and it cannot be 
brought very near the driving surface. 
451. To get the best results the heater should be stationary; it might then be of 
the most suitable material, of sufficient area of surface, and of the most efficient shape, 
irrespective of weight. The moving portion should be the cooler; it should be as 
near as possible to the heater, and of the most suitable size, shape, and weight for 
utilising the force impinging on it. The heater, or driving surface, acts as if a mole¬ 
cular wind* were blowing from it (276), principally in a direction normal to the 
surface (312). By having the heater of large size, and making it of a good conductor, 
such as silver, gold, or copper, a very faint amount of incident radiation would suffice 
to produce motion. 
In April, 1877, I communicated to the Royal Society a jneliminary note on a 
new instrument which I had made in accordance with these indications, and as it 
was essentially different in its construction and mode of action to the radiometer, 
I proposed to identify it by the distinctive name of Otheoscope (oj6ea>, I propel). 
Whilst the radiometer admits of but few modifications, such an instrument as the 
otheoscope is capable of an almost endless variety of forms. The glass envelope 
is an essential portion of the machinery of the radiometer, without which the fly 
would not move; but in the otheoscope the glass vessel simply acts as a preserver 
of the recpiisite amount of rarefaction. Carry a radiometer to a point in space 
where the atmospheric pressure is equal to, say, 1 millim. of mercury, and remove 
the glass bulb, the fly will not move, however strong the incident radiation ; but 
place the otheoscope in the same conditions, and it will move as well without the 
case as with it. 
452. The instrument described in a former paper (360), and repeated here with 
some alterations (425, fig. 17), fulfils the essential conditions of the otheoscope, and 
by allowing light to shine on the blacked mica plate instead of heating it with the 
platinum wire and battery, the vanes may be set into good rotation. 
An instrument was made of this kind, leaving out the electrical portion so as 
to work entirely by radiation. It is represented at fig. 25, A. A plate of mica, a a, 
is attached firmly to the support c, and is lampblacked on the upper side ; a fly, 
h h, with polished aluminium vanes, set at an angle of 45°, is supported by a glass 
cap on a needle point passing through a a, so that when rotating the lower edge shall 
be about a millimetre from the mica plate. Light shining on the black surface 
generates molecular pressure, which reacting on the sloping vanes drives them round 
* This movement of the molecules may be compared to the movement of the oxygen and hydrogen 
molecules when water is decomposed by an electric current. In tbe water connecting the two poles there 
is no molar movement whatever, although eight times as much matter is passing one way as the other. 
MDCCCLXXIX, 
It 
