THE PHYSICS OF A SUNBEAM. 441 
due to tliose peculiar thermic rays which are entirely indepen- 
dent of Light though associated with that principle. 
The scorching of the sun-beams, which is often observable 
in the summer and autumnal days, and which is not unusually 
associated with the appearance of blight on the leaves of plants, 
has been traced to an excess of these dark heat-rays. 
If the spectral image by which the Light and Heat spectrum has 
been produced be maintained, and we substitute a paper or a glass 
tablet covered with any of the Photographic preparations, we 
obtain a spectrum such as is represented on our drawing and 
marked Actinism. 
The Chemical tablet is blackened over some parts, and pre- 
served unchanged over other parts. Upon that space which is 
under the influence of the most Light — as indicated by the line 
across the page — there is no indication of any change, however 
long the salt of silver may be exposed to solar action. The 
space covered by the Heat rays of maximum intensity is in like 
manner preserved unchanged. Over the region of the red rays 
some chemical change goes slowly on, and frequently a red 
colour is produced; in some cases, as when the bromide of 
silver is used, rising to a fine crimson. The yellow rays have 
no chemical power, but beyond them the decomposing agency 
is exerted with increasing force ; it arrives at its maximum in 
the blue and indigo rays, and although it is weakened in force 
beyond these, the chemical power is exerted to a considerable 
distance beyond the visible spectrum, where there is no Light. 
The results which have been described naturally open the 
question of the identity of Light, Heat, and Actinism, or the 
contrary. We have seen that the maximum points differ, and 
that where light and heat are the most energetic, there the 
chemical power is the least so. 
According to the undulatory hypothesis these differences are 
explainable upon the supposition that one set of undulations 
will produce the sensation of Light, that another set establishes 
the sensation of Heat, and that a different rate of vibration 
again is the cause of Actinic phenomena. 
There is some other evidence which it is necessary to bring 
before the reader. By means of absorbent media we may 
separate these agencies and examine them in action in an 
isolated state. 
A slice of obsidian, or of black mica, will not allow any Light 
to pass through it, but it offers no obstruction to solar Heat. 
A plate of glass stained of an apple green with oxide of 
copper is perfectly transparent to Light, but it is almost 
opaque to Heat. Glass which has been stained yellow with 
oxide or chloride of silver allows a full flood of Light to pass 
