126 
THE YOUNG SCIENTIST. 
a needle on a prepared glass plate, which 
is afterwards printed like a photographic 
negative. It was suggested and tried 
more than ten years ago, but was laid 
aside for various reasons which I will not 
stop now to explain. 
Those who have the first volume of the 
Young Scientist will find in its lessons on 
photography all the apparatus and ma- 
terials named which are necessary for 
preparing the plate. It is done just as a 
photographer goes about making a nega- 
tive. The glass is first thoroughly cleaned 
and flowed with collodion, then it is 
sensitized, or coated, in a bath of nitrate 
of silver. This can be done in a shallow 
porcelain or glass tray with a silvered 
wire hook to lower and raise the plate, as 
seen in Fig. 1. 
Should the reader not care to prepare;' 
his own plates, he can get them from a 
photographer for from ten to twenty-five 
cents each. 
Cabinet size I find the most convenient 
for small studies. This will make a pic- 
ture 4 by 6 inches. 
For the first attempts the plates should 
also be varnished with a good thin nega- 
tive varnish, as the film is so delicate that 
the slightest touch will make an impres- 
sion. The sketch has now to be trans- 
ferred to the plate, and in order to have it, 
when printed, the same as the original, it 
must be reversed. If the drawing is just 
the right size, and made with a soft pencil, 
it can simply be turned over on the var- 
nish, and the back of it rubbed lightly 
with the finger, (the varnish being dry) 
Fis. 1. 
The only hint necessary about this part 
of the process is to be sure and lower the 
plate regularly, without stopping, into the 
solution. The advantage of the tray over 
the standing bath, is that you require but 
a small amount of silver solution to cover 
the plate. Sensitizing is done in a dark 
room. 
Now, if you have a camera, you can 
point it at a perfectly white screen and ex- 
pose the plate for about twenty seconds, 
and then flow over it a solution of protosul- 
phate of iron. (See Vol. 1, page 118). The 
iron will make the film opaque, and of a 
grey color. Wash well and dry, and the 
plate is ready for the picture. 
If you do not possess a camera, (it is not 
necessary) you can just open the door of 
your dark room, and let a strong white 
light fall directly upon the plate a couple 
of seconds, and then develop, with weak 
developer, as before stated. 
when the outline will be seen on the plate 
as the paper is raised. This, however, 
can only be done on varnished plates, of 
course. Another method will be treated 
of hereafter. 
The next thing of importance, is a stand 
something like a photographer's retoiich- 
ing frame. This is to rest the plate on 
while drawing with the needle, bocause 
by this process you can see the picture 
distinctly as it progresses. I will tell you 
how I made one. I took the frame of a 
broken slate and screwed strips of wood 
to the sides for legs, thus making it to 
stand like a square easel. Then I took 
two pieces of wood half an inch wide and 
a quarter of an inch thick, and just as 
long as the inside of the frame ; to each 
piece I fastened a thin strip to the narrow 
side as long as the outside measurement 
of the the frame, and with screw eyes 
near the ends attached them to the frame 
