OF THE SOLAR SPECTRUM ON VEGETABLE COLOURS. 
201 
to appear bright in opposition to clear blue sky seen through the same glass in the 
quarter of the heavens opposed to the sun, and that at noon day. The aspect of na- 
ture, indeed, when viewed through coloured glasses, is fraught with curious and inter- 
esting matter of optical remark ; but to give them their full effect they must not be 
merely applied to one eye for a few moments, as in the use of Claude Lorraine 
glasses. They should be worn as spectacles, both eyes being used, all lateral light 
carefully excluded by black velvet fringes, and their use continued till the pupil is 
fully dilated and the eye familiarized with the intensity and tone of the illumination. 
So used, not only are the ordinary relations of all lights and colours strangely and 
amusingly deranged, but contrasts arise between colours naturally the most resem- 
bling, and resemblances between those naturally the most opposed. We become 
aware of elements in the composition of tints we should otherwise never have sus- 
pected, and the singularities of idio-chromic vision which seem so puzzling, when 
related, cease to be matter of any surprise*. 
202. I shall conclude this part of my subject by remarking on the great number 
and variety of substances which, now that attention is drawn to the subject, appear 
to be photographically impressible. It is no longer an insulated and anomalous 
affection of certain salts of silver and gold, but one which, doubtless, in a greater or 
less degree pervades all nature, and connects itself intimately with the mechanism by 
which chemical combination and decomposition is operated. The general instability 
of organic combinations might lead us to expect the occurrence of numerous and re- 
markable cases of this affection among bodies of that class, but among metallic and 
other elements inorganically arranged, instances enough have already appeared, and 
more are daily presenting themselves, to justify its extension to all cases in which 
chemical elements may be supposed combined with a certain degree of laxity, and so 
to speak, in a state of tottering equilibrium. There can be no doubt that the pro- 
cess, in a great majority if not all the cases which have been noticed among inorganic 
substances, is a deoxidizing one, so far as the more refrangible rays are concerned. 
It is obviously so in the cases of gold and silver. In that of the bichromate of potash 
it is most probable that an atom of oxygen is parted with, and so of many others. 
A beautiful example of such deoxidizing action on a non-argentine compound has 
lately occurred to me in the examination of that interesting salt, the ferroses- 
quicyanuret of potassium, described by Mr. Smee in the Philosophical Magazine, 
No. 109, September 1840, and which he has shown howto manufacture in abundance 
and purity by voltaic action on the common, or yellow ferrocyanuret. In this pro- 
cess nascent oxygen is absorbed, hydrogen given off, and the characters of the result- 
* The late celebrated optician Mr. Troughton, who was a remarkable instance of this sort of vision, in- 
formed me that he could not distinguish the scarlet coats of a regiment of soldiers from the green turf on which 
they were drawn up, nor ripe cherries from the leaves of the tree which bore them. His eyes, however, were 
perfectly sensible to rays of every refrangibility as light, but the spectrum afforded him only the sensations of 
two colours, which he termed blue and yellow ; pure red and pure yellow rays exciting in his mind the same 
sensation. 
2 D 
MDCCCXLir. 
