36 



Mr. W. Crookes on 



[Nov. 21, 



Instead of allowing the clear screens to freely move on a pivot, an 

 instrument was made in which the screens could be fixed beforehand 

 in any desired position in respect to the blacked disks. It was then 

 found that with the screens close to the blacked sides of the vanes 

 the fly rotates very slowly in the negative direction, stopping 

 altogether when the candle is moved five or six inches off. With 

 the screens 1 millim. from the black surface the direction is negative 

 and the speed at its maximum. When the screens and disks are 

 7 millims. apart a position of neutrality is attained, no movement 

 taking place. When the distance is further increased, positive rota- 

 tion commences, which gets stronger as the screens approach the 

 bright sides of the disks, where the positive rotation is at its maxi- 

 mum. The author adduces reasons for considering that the negative 

 rotations here observed are caused by the warming up of the black 

 surface by radiation falling direct on it, through the clear mica 

 screen, and the deflection backwards of the lines of molecular pressure 

 thereby generated. 



The action of these radiometers being complicated, owing to the 

 surfaces of the vanes being different in absorptive power, another in- 

 strument was made in which the vanes were of polished aluminium, 

 perfectly flat and symmetrical with the bulb. The screens were of 

 clear mica movable in respect to the vanes, and at right angles 

 to their surface. When exposed to the light of a candle it was 

 found that with the screens brought up close to the disks, the rotation 

 was as if the unscreened side were repelled ; at an intermediate posi- 

 tion there was neutrality. Explanations are given of these move- 

 ments, but without the illustrative cuts they would be unintelligible. 



Experiments on radiometers having movable screens interposed be- 

 tween the vanes and the bulb are next given, and these are followed 

 by a long series of experiments on the influence of movable screens on 

 radiometers with cup-shaped metallic vanes, the screens being varied 

 in shape, and position in respect to the plane of rotation, as well as in 

 respect to the distance from the vanes. 



A similar series is given with metallic cylinders as vanes, and from 

 the behaviour of the latter kind of radiometer, an explanation is given 

 of the various movements previously obtained. It is found that 

 when the screen touches the convex surface of the vanes the rotation 

 under the influence of light is always positive. It commences at a 

 low exhaustion, increases in speed till the rarefaction is so high that an 

 ordinary radiometer would begin to lose sensitiveness, and afterwards 

 remains at about the same speed up to the highest rarefaction yet 

 obtained. At any rarefaction after 87 M (millionths of an atmo- 

 sphere) there is a neutral position for the screen. When it is on the 

 concave side of this neutral position the direction of rotation is 

 positive, and when on the convex side of the neutral position it is 



