8 REPORT—1843. 
experiment which Sir D, Brewster was so kind as to make at my request, is, so far as 
it goes, against this supposition ; and if further experiments should tend the same 
way (as most probably they will) it will follow that the unsymmetrical effects of re- 
flexion do not arise from any cause which penetrates the interior of the crystal, but 
only from some peculiar structure of its surface. When the action of the crystal upon 
light is supposed to be symmetrical round its axis, the law in question reduces itself 
to that of Huygens; and in the case of a biaxal crystal, when the phenomena are 
supposed to be symmetrical with respect to three rectangular axes, the law coincides 
with that of Fresnel. 
On the Action of Two Blue Oils upon Light. By Sir D. BRewsTER. 
Having lately received, through the kindness of Dr. Gilbert, two remarkable oils of 
a deep blue colour, namely, the vil from the Matricaria chamomilla, and that from the 
Achillea millefolia, I was desirous of ascertaining the nature of their action upon the 
solar spectrum. Without entering into details respecting the general action of these 
oils upon the different coloured portions of the spectrum, I shall confine myself to a 
slight notice of their specific action, in which they differ from all the various bodies 
which I have yet examined. 
Between the two lines A and B of Fraunhofer’s map of the spectrum there are two 
groups of lines shown in that map. The two oils absorb the light in these portions 
more powerfully than the portions adjacent to them. No other fluid or solid body 
on which I have hitherto made experiments acts in a similar manner; but what is 
very remarkable, the earth’s atmosphere exercises a similar action when the sun’s 
light passes through its greatest thickness at sunrise or sunset. 

Notice of a remarkable Photographie Process by which dormant Pictures are 
produced capable of development by the Breath or by keeping in a Moist 
Atmosphere. By Sir Jonny HERSCHEL. 
If nitrate of silver, specific gravity 1200, be added to ferro-tartaric acid, specific 
gravity 1:023, a precipitate falls, which is in great measure redissolved by a gentle 
heat, leaving a black sediment, which being cleared by subsidence, a liquid of a pale 
yellow colour is obtained, in which a further addition of the nitrate causes no turbid- 
ness. When the total quantity of the nitrated solution added amounts to about half 
the bulk of the ferro-tartaric acid, it is enough. ‘The liquid so prepared does not alter 
by keeping in the dark. 
Spread on paper and exposed wet to the sunshine (partly shaded) for a few seconds, 
no impression seems tohave been made, but by degrees (although withdrawn from the 
action of the light) it developes itself spontaneously, and at length becomes very in- 
tense. But if the paper be thoroughly dried in the dark (in which state it is of a very 
pale greenish yellow colour) it possesses the singular property of receiving a dormant 
or invisible picture ; to produce which (if it be, for instance, an engraving that is to 
be copied) from thirty seconds to a minute’s exposure in the sunshine is requisite. It 
should not be continued too long, as not only is the ultimate effect less striking, but a 
picture begins to be visibly produced, which darkens spontaneously after it is with- 
drawn, But if the exposure be discontinued before this effect comes on, an invisible 
impression is the result, to develope which all that is necessary is to breathe upon it, 
when it immediately appears and very speedily acquires an extraordinary intensity and 
sharpness as if by magic. Instead of the breath it may be subjected to the regulated 
action of aqueous vapour, by laying it in a blotting-paper book of which some of the 
outer Jeaves on both sides have been damped, or by holding it over warm water. 
Many preparations both of silver and gold possess a similar property, in an inferior 
eat a none that I have yet met with to anything like the extent of that above 
described. 
eee 

