OF THE SOLAR SPECTRUxVI ON VEGETABLE COLOURS. 
189 
colour are separated, by destroying the one and leaving the other outstanding. The 
older the paper, or the tincture with which it is stained, the greater is the amount of 
this residual tint. 
2nd. The action of the spectrum is confined, or nearly so, to the region of it occu- 
pied by the luminous rays, as contra-distinguished both from the so-called chemical 
rays, beyond the violet, which act with the chief energy on argentine compounds, but 
are here for the most part ineffective, on the one hand, and on the other, from the 
thermic rays beyond the red, which appear to be totally so. Indeed, I have hitherto 
observed no instance of the extension of this description of photographic action on 
vegetable colours beyond, or even quite up to the extreme red. 
170. Besides these, it may also be observed that the rays effective in destroying a 
given tint, are, in a great many cases, those whose union produces a colour comple- 
mentary to the tint destroyed, or at least one belonging to that class of colours to 
which such complementary tint may be referred. For example, yellows tending to- 
wards orange are destroyed with more energy by the blue rays ; blues by the red, 
orange, and yellow rays ; purples and pinks by yellow and green rays. 
171. These are certainly remarkable and characteristic peculiarities, and must 
indeed be regarded as separating the luminous rays by a pretty broad line of chemical 
distinction from the non-luminous ; though whether they act as such, or in virtue of 
some peculiar chemical quality of the heat which accompanies them as heat, is a point 
which the experiments on guaiacum, above described, seem to leave rather equivocal. 
In the latter alternative, chemists must henceforward recognize differences not simply 
of intensity, but of quality in heat from different sources; of quality, that is to say, 
not merely as regards degree of refrangibility or transealescence, but as regards the 
strictly chemical changes it is capable of effecting in ingredients subjected to its in- 
fluence. 
172. As above stated, these peculiarities, at least the first two, obtain almost uni- 
versally. Exceptions, however, though very rare, do occur, as will be more particu- 
larly mentioned hereafter. The third rule is much less general, and is to be interpreted 
with considerable latitude; but among its exceptions I have been unable to detect 
any common principle capable of being distinctly enunciated. 
173. Lastly, it requires to be expressly mentioned, that the habitudes of the colours, 
both of the flowers and leaves of plants, with relation either to white light or to the 
prismatic rays, vary materially with the advance of the season, and perhaps also with 
the hour of the day at which they are gathered. Generally speaking, so far as I have 
been able to observe, the earlier flowers of any given species reared in the open air 
(provided they are well ripened, i. e. the colour fully developed) are more sensitive 
than those produced even from the same plant, at a late period in its flowering, and 
have their colours more completely discharged by light. As the end of the flowering 
period comes on, not only the destruction of the colour by light is slower, but residual 
tints are left which resist obstinately. A very remarkable case of this kind was no- 
