164 
NATURE 
| Dec. 15, 1881 
with one kind of electricity. These sub-molecules, the sub-iodide 
and the sub-chloride, are unsaturated compounds, and are ready 
to join hands with any body for which they have the least affinity. 
Suppose then we have a solution of a silver salt, say of silver 
nitrate, and that we introduce some body which will precipitate 
that silver in a metallic state, and suppose again that one atom 
of silver as it is precipitated is charged with one kind of elec- 
tricity, it is quite within the range of probability that the silver 
atom, as it is precipitated in the solution, may be attracted 
by the sub-iodide of silver when charged with another kind of 
electricity, and so form a new molecule as it were. This built- 
up molecule will not be fully satisfied, but probably have an 
excess of the opposite kind of electricity, with which the sub- 
iodide was charged, and the silver atoms which were oppo- 
sitely charged to those first attracted would in their turn be 
attracted, and so on until the image is, asit were, built up on the 
small quantity of sub-iodide first formed by the action of light. 
In the case of silver bromide we have the same thing happening. 
We have sub-bromide of silver formed, which is represented by 
one atom of silver, and one of bromine, Ag’ Br, and it is quite 
within the range of possibility that another body may be 
brought in contact with that, which, being charged with elec- 
tricity opposite in polarity to that with which this molecule 
is charged, may attract away, as it were, the bromine, and thus 
leave the metallic silver itself behind. This is what happens 
really in the case of what we call alkaline development. In the 
first case we had acid development, or an image built up by the 
deposition of silver from silver nitrate, and in the other abstrac- 
tion of bromine from the small quantity of sub-bromide of 
silver whichis formed bythe action of light. The image so 
built up, however, would scarcely be apparent, since the metallic 
silver thus formed would be inappreciable to the eye. Another 
phenomenon seems, however, to present itself, and that is, that 
the atoms of metallic silver, and of the unaltered bromide of 
silver, are oppositely charged with electricity, and combine to 
form fresh sub-bromide of silver ; these new molecules of sub- 
bromide are reduced, and so the action goes on till an image 
is built up, each molecule of sub-bromide originally formed by 
the action of light forming a nucleus for the reduction. If 
instead of forming the iodide and bromide of silver in collodion 
films, as is usually the case now, we form iodide and bromide 
of silver on a metallic silver plate by allowing iodine and 
bromine vapour to have access to it, and if, after exposure to | 
light, we allowed mercury vapour to act upon it, then the same 
kind of action would take place, the condensed mercury vapour 
would be attracted to those points which bad been acted upon 
by light. 
known as daguerrotype. 
I now propose to show you a practical demonstration of the 
two methods of development of which I have shown you the 
outlines by diagrams, First, I will ask you to notice the part of 
the spectrum which the silver iodide plate cuts off. Thaye placed 
That is the earliest form of photography, and was | 
such a plate in front of the slit of the lamp, and you will see the | 
blue is cut off. Todide like chloride cuts off the blue rays, but 
with more intensity than the chloride. 
most effective in altering iodide of silver. I have a photo- 
graphic spectrum apparatus placed in position. We have the 
Siemens light as before, a collimating lens (about which I need 
not enter into details), a prism through which the light has to 
pass from the slit, and here we have a lens and an ordinary 
camera. I propose to place a plate coated with silver iodide in 
the spectrum of the arc, and another coated with bromide, and 
then develop them if possible on the screen before you by the 
two methods of development. The first plate we expose is an 
ordinary wet plate, z.e. we have a collodion film which covers | 
the plate, and this collodion film contains iodide of silver, and it 
“is moistened with nitrate of silver. We will expose that to the 
spectrum fora second. I have in front of the slit at present a 
solution of bichromate of potash, which cuts off the blue, and 
therefore the light passing through would have no effect on the 
plate. I withdraw the front of the slide, and give a very short 
exposure. 
a developing solution, that is, something which will precipitate 
metallic silver from the soluble nitrate of silver ; we will place 
it upon the stand of the lantern, B, and by means of the lens G 
we shall see a reversed image of part of the cell on the 
sereen. I next place a piece of yellow glass, FE, in front of 
the Jantern lens. I take the plate and simply immerse it in 
the solution, and by degrees you will see a blackening take 
Now we will take a cell, F, containing what we call | 
Where there is absorp- | 
tion there alone can work be done, the blue rays are therefore | 
place. I am afraid the film is a little too intense—it is now 
coming out more rapidly and more vigorously ; and here we have 
the image of the blue end of the spectrum perfectly developed, I 
will get my assistant to take charge cf this, and in the mean- 
while I will throw on the screen another spectrum which was 
taken in a similar manner. 
Now we will try the other mode of development, which I hope 
will be more visible to the audience than the one we have tried. 
We will use the Siemens light again to form the spectrum. We 
will keep the yellow solution in front to cut off the blue rays 
whilst focussing it, and then I am perfectly safe. Then we will 
remove the yellow solution, and give a very short exposure, and 
we will develop by the alkaline process. We will use the 
same apparatus as before for developing. We have here a cell 
containing the liquid which has a great affinity for bromine, and 
I have no doubt we shall find that the solution will take away 
the bromine and leave metallic silver behind. It will probably 
be rather slower than the other in appearing. The plate is now 
placed in the cell, and we see the lines of the spectrum are 
appearing, and finally the image is fully out. I now withdraw 
the yellow glass. ; 
You may ask how it is I can afford to let the light fall on th 
plate without causing a further deposition. The fact is, this solutio 
itself is coloured red, and therefore the light passing through it has 
no effect on the bromide. I daresay by and by, when those pictures 
are thrown on the screen, you will be able to see what kind of 
spectrum we have got. You will remember, then, that we have 
in these methods of development which I have endeavoured to 
bring before you two kinds of chemical action—one a physical 
action of attraction exercised by sub-iodide of silver, the other 
a chemical one by the taking away of bromine from the silver 
salt. 
‘The first mention that we have of a photographic spectrum 
in the red or below the red was in the year 1839, when Dr. Draper 
published a paper saying that he had been able to observe certain 
lines in the solar spectrum below the limit of the red. I propose to 
show you on the screen a copy of his photograph, and explain 
how it was taken. Dr. Draper proceeded in this way. He 
took a daguerreotype plate, exposed it to light first, then exposed 
it to the spectrum. In Dr, Draper’s spectrum we have the 
whole of the blue and violet part of the spectrum delineated, but 
in part of the green and all of the yellow there isa blank, but be- 
1 The plate was developed by ferrous oxalate. 
