[66 PRODUCTION OF SPECTRAL APPEARANCES. 



713. Much of this effect is due, as I have endeavoured in the paper above quoted to 

 show, to a direct escape of dark rays by a process analogous to radiation ; but much 

 also is due to a hitherto unknown power, possessed by electro-negative gases and va- 

 pours, which tends to bring about the same results. So powerfully, indeed, does this 

 cause operate, that, as I have said, for a length of time I attributed all the phenomena 

 to it. 



714. I proceed now to describe some simple experiments which will bring this 

 matter clearly before the reader. 



715. Take a bromo-iodized silver plate, expose it to the light of the sky or lamplight 

 for a length of time sufficient to brown it sensibly and uniformly all over. In this state, 

 if it were placed in the vapour of mercury, it would solarize or blacken in every part. 

 But, before mercurializing, treat it as follows : Lay upon it a fragment of glass, a piece 

 of metal, or any other object ; immerse it for a second or two in a box containing the 

 vapour of iodine ; withdraw it, remove the little object, and mercurialize forthwith ; 

 and now you will find a perfectly-formed, black spectral impression of the object, 

 whatever it was, powerfully brought out by the mercury vapour ; but on all those 

 parts to which the iodine vapour has had access the mercury will not adhere, but the 

 phenomenon will take effect as though the plate had never been exposed to the light, 

 except on those portions on which the object, whose spectral image appears, was laid. 



716. From this it would seem that the vapour of iodine has the quality of detithon- 

 izing a surface that has been changed by light. 



717. The same process may be conducted so as to give a still more striking result. 



718. Employing a prepared bromo-iodized plate, as before, expose it to any uniform 

 source of light for such a length of time that, if it were mercurialized, it would whiten 

 uniformly, and exhibit the aspect of an ordinary white Daguerreotype. Treat it as 

 before, by placing on it any object, pass it into the vapour of iodine, remove the object, 

 and mercurialize ; and now a spectral appearance of that object, of a dense white as- 

 pect, will emerge, the remainder of the plate being quite black and in the condition 

 of the shadows of a Daguerreotype, that is, as though it had never been exposed to 

 the light. 



719. In order to obtain a clear idea of what passes under the foregoing circumstances, 

 I made the following trial. 



Upon a plate prepared and deeply tithonized, as has been said, I laid a double 

 convex lens of about two inches focus, and exposing the plate, with the lens upon it, 

 to the vapour of iodine, and then removing the lens, I mercurialized. A deep-blue 

 spectral image emerged, of less diameter than the lens, but, like it, of a circular form, its 

 circumference being marked by as sharp a line as if it had been drawn by a pair of 

 compasses. Indeed, it looked as distinct and as sharp as if a blue wafer had been laid 

 on the plate. 



720. In several successive trials I found that tne magnitude of this spectre dimin- 

 ished as the time of exposure to the vapour had been prolonged. 



721. Next, I repeated the same trial, using the plate and lens as just described ; 

 but immersing the plate in the vapour of bromine instead of that of iodine, a still 



