[ 87 ] 
IV. On Repulsion resulting from Radiation .—Part VI 
By William Crookes, F.R.S., &c. 
Received June 27,—Read November 21, 1878. 
Contents. 
Par. 
Direction of the lines of pressure inside the 
radiometer. 887 
Effect of mica screens on the rotating vanes 388 
Action of movable and fixed screens. 391 
Neutral position for screen. 400 
Screens of good and of bad conductors. 407 
Neutral position varies with degree of ex¬ 
haustion . 409 
Theoretical lines of molecular pressure .... 415 
Verification of theory by means of small 
exploring fly. 418 
Action of heat applied inside the radiometer 425 
The turbine radiometer. 434 
Influence of size of bulb on rotation. 437 
Par. 
Spiral radiometers . 439 
Reaction against inner surface of glass. 441 
Rotation of the case of the radiometer. 442 
Radiometers with fixed vanes. 447 
Obliquity thrown from fly to case. 449 
The otheoscope. 450 
Rotation of a disk on its axis. 454 
Thickness of layer of molecular pressure .. . 455 
Attraction at atmospheric pressure . 457 
Repulsion at atmospheric pressure. 461 
Experiments with bar photometer. 476 
Measurement of the force of repulsion ...... 478 
Point of maximum molecular pressure. 481 
DIRECTION OF THE LINES OF PRESSURE INSIDE THE RADIOMETER. 
387. Although the general character of the reactions which cause repulsion under the 
influence of radiation is now understood, much light may be thrown on the subject by 
an experimental examination of the direction and strength of the lines of pressure 
inside the case of a radiometer on which light is allowed to fall. Radiation will pass 
almost unimpeded through a very thin, colourless and transparent substance such as 
mica, but molecular pressure or stress is arrested by such a body (232). By introducing 
fixed or movable screens in various parts of the case of a radiometer, the direction of 
pressure can be determined at will, and its force can be modified in many ways, whilst 
all the other conditions of the experiment remain unchanged. 
In the present Part I propose to give the results of a long series of experiments 
on the action of thin mica screens in modifying the movements of the fly of a radio¬ 
meter ; I shall examine the action of the residual gas, the action of the sides of the 
glass case, and the applicability of the information so afforded to the construction of 
instruments of greatly increased sensitiveness for the purposes of research and illus¬ 
tration ; and I shall also describe other experiments which have been tried from 
