540 
ME. W. CROOKES ON KEPULSION BESETTING EEOM BADIATION. 
easy, owing to the fluorescence of the card and other surfaces on which stray rays fell. 
The spot of light moved two divisions, which were increased to five when the invisible 
rays were further concentrated by a quartz lens. The interposition of the iodine-cell 
cut off the whole of the action. The alum plate cut off about half of the action, but 
scarcely more than would have been cut off had a piece of colourless glass ‘of the 
same thickness been interposed, and it must be remembered that the alum plate has 
glass and Canada balsam on each side. 
111. A similar experiment with the solar spectrum gave the following deflections, 
glass prisms being used : — 
Ultra-red 2 
Extreme red 6 
Orange 5 
Green 4 -5 
Indigo 3 ’5 
Ultra-violet 2 
Although I give the number of divisions shown by the luminous index, I attach 
little importance to them, as quantitative measurements. They are only single obser- 
vations, and were taken before I had succeeded in getting any thing like the same 
sensitiveness I can now attain in the apparatus. As illustrations of the fact, however, 
that the more refrangible rays of the spectrum act as well as the lower rays, they may 
be taken as trustworthy*. 
112. In my former paper on this subject I have already mentioned in detail that at 
a certain point of rarefaction there is neither attraction nor repulsion when radiation 
falls on the movable index (30, 43, 47, 66). I have long tried to ascertain the law 
governing the position of this neutral point. My results are not yet ready for publi- 
cation ; but they are shaping themselves in order, and will, I trust, lead to a true 
explanation of the cause of these phenomena. 
The barometric position of the neutral point dividing attraction from repulsion varies 
according to circumstances ; among these may be mentioned the density of the sub- 
stance on which radiation falls, the ratio of its mass to its surface, its radiating- and 
conducting-power for heat, the physical condition of its surface, the kind of gas filling 
the apparatus, the intensity of radiation, and the temperature of the surrounding atmo- 
sphere. 
When the surface exposed to radiation is pith, the neutral point is somewhat low. 
I have had it vary between 50 millims. and 7 millims (30) below a vacuum. It is, how- 
ever, impossible to ascertain exactly ; for a point of rarefaction can be obtained at which 
the warm fingers repel, and incandescent platinum attracts. With a heavy metal in 
the form of a sphere, so as to expose the smallest surface in proportion to the mass, I 
* Every thing is ready to try a series of experiments with the solar spectrum, as soon as sunshine is avail- 
able. The results shall he communicated in a subsequent paper. 
