SCIENTIFIC SUMMARY. 
323 
meter, whatever form it may assume, the glass glohe is an essential part of 
the apparatus. Without the glohe the fly would never turn, and the most 
probable view of the action of the radiometer — the view to which the in- 
ventor himself assents — is that rotation must be caused by the excess of 
molecular pressure between the fly and the enclosing vessel. In the otheo- 
scope, on the contrary, the glass bulb serves merely as an envelope to en- 
close the rarefied air, and is by no means an essential part of the machinery. 
In fact, an otheoscope has been constructed without any envelope whatever. 
Let a radiometer and an otheoscope be removed to a position in space where 
the air is extremely attenuated, so that the atmospheric pressure is perhaps 
not more than a millimeter of mercury, and then remove the glass case from 
each instrument ; the fly of the radiometer immediately would becom e 
stationary, though there might be abundant solar radiation, but the fly of 
the otheoscope would whirl round just as merrily as before. The first form 
of otheoscope described by Mr. Crookes (“ Chemical News,” May 4, 1877), 
consists of a four-armed fly, each carrying a vane of thin clear mica. At 
one side of the glass bulb which encloses the apparatus there is a vertical 
plate of mica blackened on one face, and so placed that each vane closely 
approaches it as the mill rotates. If light be allowed to fall only upon the 
clear vanes, no motion is produced ; but if the light shine upon the black 
plate the vanes instantly begin to rotate, as though repelled by a molecular 
wind blowing from this surface. The movement is therefore produced by 
pressure generated on a fixed part of the apparatus, by which the moveable 
portion is propelled. As this driving-surface is stationary, it is not re- 
stricted in weight, size, or shape ; and hence the modifications of which 
the otheoscope admits are well-nigh endless. 
The Cycloscope . — It is well known that if a mirror be attached to a 
vibrating tuning-fork, and a point of light which moves uniformly in a plane 
at right angles to that in which the fork is vibrating be reflected from this 
mirror, the image will bean ordinary single wave. Again, if a ’series of 
luminous points move uniformly with such velocity that a point passes over 
two intervals during an odd number of vibrations of the fork, the two waves 
overlap and produce a double figure of the form of a series of figures-of-eight. 
Extending these principles, Professor McLeod and Lieutenant Gf. S. Clarke 
have recently constructed an ingenious apparatus which has been described 
before the Royal Society under the name of the cycloscope. Equidistant 
perforations are made in a circle on a disc, which is attached to a rotating 
axis, and the light passing through these apertures falls upon a vibrating 
tuning-fork of known period, whence it is reflected on to a screen : and 
from the shape of this reflected image the rate of rotation can be deduced. 
Hence the cycloscope promises to become of much value in determining the 
speed of machinery. On the contrary, if the speed at which the cylinder 
rotates be known, the pitch of the tuning-fork may be ascertained. — Proc. 
Poy. Soc., April 19, No. 180, p. 157. 
