RAYS OF THE SOLAR SPECTRUM ON PREPARATIONS OF SILVER, ETC. 5 
that a saturated brine dissolves this substance pretty freely, producing 1 a solution of 
an intensely bitter metallic taste, from which the greater part, if not the whole chlo- 
ride, is thrown down by copious addition of water. I cannot say, however, that I 
have found much satisfaction in the use of salt as a fixing material. 
17. Ilydriodate of potash, if the right strength be hit, succeeds well; but to do so 
is not very easy. Moreover, it gives a yellow tint to the ground of the picture, highly 
unfavourable to transfers. The very curious property possessed by this salt, when 
applied of a certain strength, of rendering photographic paper, blackened by expo- 
sure to light, susceptible of being again whitened by the same agent, has been noticed 
by Lassaigne, Mr. Talbot and others, and was early encountered by myself. My at- 
tempts to apply this property to the production of a positive photographic paper, (i. e. a 
dark ground, on which the action of the light shall impress a bright image,) though oc- 
casionally successful, and in one or two instances remarkably so, have yet not enabled 
me to state any precise proportions of ingredients, and mode of applying them, which 
shall certainly produce a paper of the required character possessed of any degree of 
permanence. In fact, nothing can be more variable and capricious than the results 
obtained according to the different intensities of the solutions applied — the qualities 
of the paper — the degree of darkening induced on the paper before the application 
of the ioduretted solution — the state of the paper as to moisture or dryness, and other 
circumstances. Nor will this appear strange when the complicated nature of the 
action itself, when analysed as will be explained in the subsequent part of this paper, 
is considered. It will there be shown that the total effect of a ray of white light on 
iodic preparations is in fact the difference of two opposing actions, either of which 
is susceptible of being exalted or enfeebled at pleasure by circumstances under our 
command, indeed, in a general way, but difficult to reproduce exactly at our pleasure. 
When these opposing actions exactly neutralize each other, the paper is insensible. 
When either preponderates, it is positive or negative in its character according to 
that of the preponderant action ; nay, it majr at one and the same moment be po- 
sitive to light incident under certain circumstances, insensible under others, and ne- 
gative under a yet different illumination. In reference to practical applications, 
however, I ought to mention that others appear to have met with better success than 
I have done, perhaps from operating on a larger scale, in which the proportions of 
ingredients may be more certainly determined and adhered to. A positive paper of 
this nature is actually prepared for sale by Mr. Robert Hunt of Devonport, specimens 
of which he has been so obliging as to send me, and which certainly gives results of 
great promise in this line. 
18. A weak solution of ferrocyanate of potash renders argentine papers insensible 
to light; and as it is readily procured in the shops, which the hyposulphites are not, 
it would seem to be a very convenient fixing material. Unfortunately, however, it 
cannot be used for this purpose, since pictures fixed with it, though perfectly insen- 
sible to the strongest sunshine, are gradually but very slowly obliterated in the dark. 
