368 M. C. Lea on certain phenomena of Light. 
gelatinous alumina ina film of collodion. This is accomplished 
as follows ; 
A salt of aluminum soluble in alcohol is selected, the bromid 
is that which I have used. drops of strong alcoholic 
solution of the aluminum compound is added to ordinary plain 
collodion (made by dissolving a drachm of photographic pyroxy- 
in in six ounces of ether and as much alcohol) and to this a 
very few drops of strong liquid ammonia are added, and, after 
shaking, a little ether. After standing a day, this mixture can 
be extended over glass, and after it is set, the bromid of ammo- 
nium is to be washed out, and the film dried. oe 
Such a plate placed in the path of the ray colors it bright 
orange yellow. If it be laid upon the mirror so that the ray 
ses through it twice, viz: before and after reflection, the 
beam will be colored orange red. rer 
Thin strata of bromid of silver upon glass produce a sim > 
lar effect. : : 
y own views as to the nature of this coloration which will 
be presently explained, led me to the conclusion that a pre- 
cisely similar result would be produced by any substance of a 
sufficiently fine state of division, irrespective of its color. 
Experiment confirmed this expectation and led to a remarkably 
son of their two maxima of absorption. Copper was selected 
which is wholly free from this objection, and films of hy drated 
oxyd of copper were obtained by preparing a saturated alcoholic 
solution of chlorid of copper, and by treating it precisely 
of alumina. 
was 
that 
: ight transmitted through it, become salmon-t 
