38 J. W. Draper—Distribution, ete., in the Spectrum. 
mide of potassium, or fluoride of potassium, is used. In all 
these the reversing action is from F to H, and has its maximum 
somewhere about G. at is, the reversing action coincides 
with the direct action; there is no protection in the lower por- 
tion of the spectrum as in the daguerreotype. The effect is 
altogether due to the change of composition of the sensitive 
film rdinarily it contains free nitrate; now it contains free 
iodide, chloride, &ec. 
he silver compounds of collodion absorb the radiations fall- 
ing on them which are capable of producing a photographic 
effect. Yet sensitive as it is, collodion is very far from having 
its maximum sensitiveness, as is shown by the following experi- 
ment, which is of no small interest to photographers. I took 
five dry collodion plates, prepared by what is known as the 
tannin process, and having made a pile of them, caused the rays 
of a gas flame to pass through them all at the same time. On 
developing it was found that the first plate was strongly im- 
pressed, and the second, which had been behind it, apparently 
uiteas much. Even the fifth was considerably stained. From 
this it follows that the collodion film, as ordinarily used, absorbs 
only a fractional part of the rays that can affect it. Could it be 
to produce a maximum effect. 
Though the silver iodide is affected by radiations of every 
refrangibility, it is decomposed so that a subiodide results only 
by those of which the wave-length is less than 5000. If in 
presence of metallic silver, as on the daguerreotype tablet, the 
iodine disengaged unites with the free silver beneath. The 
rays of high refrangibility occasion in it chemical decomposi- 
tion, those of less rebrangitiey: physical modification. In the 
language of the older theories of actino-chemistry, this sub- 
stance may be said to exert a selective absorption. In this it 
illustrates the general principle, that it depends on the nature of 
the ponderable material presented to radiations which of them 
shall be absorbed. 
(To be continued.) 
