1876.] 
Mechanical Action of Light. 
237 
of light had to pass, with cotton-wool and large bottles of 
water. A heliostat reflected a beam of sunlight in a con- 
stant direction, and it was received on an appropriate ar- 
rangement of slit, lenses, prisms, &c., for projecting a pure 
spectrum. Results were obtained in the months of July, 
August, and September; and they are given in the figure 
(Fig. 5) graphically as a curve, the maximum being in the 
ultra-red and the minimum in the ultra-violet. Taking the 
maximum at 100, the following are the mechanical values of 
the different colours of the spectrum 
Ultra-red ......... 100 
Extreme red 
Red . . . 
Orange . 
Yellow . 
Green 
Blue . . 
Indigo 
Violet 
Ultra-violet 
85 
73 
66 
57 
41 
22 
8± 
6 
5 
A comparison of these figures is a sufficient proof that the 
mechanical action of radiation is as much a function of the 
luminous rays as it is of the dark heat-rays. 
The second question, namely, “ What influence has the 
colour of the surface on the action ? ” has also been solved 
by this apparatus. 
In order to obtain comparative results between discs of pith 
coated with lamp-black and with other substances, another 
torsion apparatus was constructed, in which six discs in 
vacuo could be exposed one after the other to a standard light. 
One disc always being lamp-blacked pith, the other discs 
could be changed so as to get comparisons of action. Calling 
the action of radiation from a candle on the lamp-blacked 
disc 100, the following are the proportions obtained 
Lamp-blacked pith 
Iodide of palladium . 
Precipitated silver 
Amorphous phosphorus 
Sulphate of baryta . 
Milk of sulphur . . 
Red oxide of iron 
Scarlet iodide of mercury and coppei 
Lamp-blacked silver 
White pith . . . 
Carbonate of lead 
Rock-salt . . . 
Glass . . . . 
100 
87'3 
56 
4 ° 
37 
28 
22 
18 
18 
13 
6*5 
6*5 
