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POPULAR SCIENCE REVIEW. 
in rotating pass the plate, leaving a space of about a milli- 
metre. Light falling on the clear vane produces no motion, 
but if shining on the black surface the fly rotates as though a 
wind were issuing from this surface. In the latter case a large 
horizontal disc is made to revolve by the molecular disturbance 
on the surface of inclined metallic vanes, which are blackened 
on both sides so as to absorb the maximum amount of radiation 
or inclined aluminium vanes are driven by molecular disturbance 
from a thick black mica disc below blowing through them.. 
Both these last work on the smoke-jack principle. Six forms 
of otheoscope and thirteen new forms of radiometer are described 
in this paper ; all, however, vary chiefly in mechanical arrange- 
ment, but not in the fundamental principle upon which they 
are based. 
Here what may be termed the official history of the radiometer 
may be said to end for the present. But of course it has formed the 
subject of many popular lectures, and of articles in semi-scientific- 
and literary periodicals. Of the former the most remarkable 
was a lecture given by Mr. Crookes himself at the Boyal Insti- 
stitution ; and in the latter class may be named memoirs in the 
journal u Engineering” and in the 66 Nineteenth Century.” To 
the latter paper a contribution was also sent by Dr. Carpenter, 
in which, while commenting on Mr. Crookes’s views with respect 
to spiritualism, he appeared to depreciate that gentleman’s phi- 
losophical capacity on the ground that he had at first attributed 
the motion of the radiometer to the direct action of light. Pro- 
fessor Carey Foster commented on this article in his address as 
chairman of the Physical branch of the British Association., 
Dr. Carpenter replied by a letter in the columns of “ Nature ; ” 
and hence arose a long and rather acrimonious controversy, in 
which all the gentlemen hitherto named, and some others, were 
for some months engaged. It will be seen from the narrative 
given above that the inventor certainly leaned at first to the 
Light hypothesis, which many experiments seemed to favour. 
But he certainly did not commit himself to it. Nor indeed, if 
he had done so, would it have been, as Dr. Carpenter seems to 
think, any evidence of logical imbecility and almost of mental 
obliquity. Oscillations of opinion around a new, startling, and 
unexplained fact are not strictly errors; but only provisional 
and perhaps feeble guesses, easily cleared away by a more for- 
cible generalization. There is no doubt, for instance, that 
Faraday attributed the rotation of the plane of polarized light 
under the influence of an electro-magnet to some action of 
magnetism on light itself, and not, as has since been shown to be 
the case, on the molecular arrangement of the particles of heavy 
glass through which the beam was made to pass. But it never 
occurred to anyone to suggest that he in any way sank from his- 
