48 
THE YOUNa SCIENTIST. 
Simple Lessons in the Art of Photo- 
graphy. 
THE CAMERA. 
rpHE camera obscura, or darkened cham- 
J- ber, is the invention of Baptista Porta, 
of Padua. Its principle will be easily un- 
derstood by the very simple experiment of 
darkening a room and admitting a single 
pencil of light through a small hole in the 
window shutters. If a piece of paper is 
held at a little distance from this aperture, 
the figure of external objects will be seen 
delineated upon it. A lens placed in the 
hole will render the picture more distinct 
and brilliant. 
A dark box may be substituted for the 
dark room, and the effect will be the same. 
Fig. 1. 
Fig. 1 represents such a box, A being the 
lens, and the opposite end the ground glass 
or oiled paper. 
In the ordinary camera used for tracing 
the outlines on ground glass with a lead pen- 
cil, a mirror is introduced, which throws 
the picture upwards on the ground glass, 
placed horizontal, for the sake of greater 
convenience in drawing. Fig. 2 shows the 
section of a sketching camera. C is the 
Fig. 2. 
lens fastened in a sliding tube; h is the 
mirror placed accurately under an angle of 
forty-five degrees, or half a right angle; A is 
a piece of ground glass, which is removable. 
The focus of the lens must be equal to the 
distances A to h, and h to C. If, for in- 
stance, A is 3 inches, and 0 & 6 inches^ 
the focus of the lens for such a camera 
should be about 9 inches. The longer the 
focus of the lens, the larger is the picture 
on the ground glass. A lid may be used to 
exclude the light during sketching, but a 
piece of dark cloth one yard square will do 
it more effectually. 
In photographic cameras the mirror is 
omitted, since its absorption of light would 
necessitate a longer exposure. Fig. 3 repre- 
sents the simplest form of a photographic 
Fig. 3. 
camera. It consists of an outer box. A, and 
an inner box, h\ the latter slides in the 
former. The rear end, C, is especially 
adapted for the reception of the plate- 
holder. The plate-holder, Fig. 4, consists 
of a square frame, with a door on the rear 
Fig. 4. 
end, and a slide moving in grooves on the 
front side. The sensitized plate is placed 
on silver wires stretched across the corners 
of the frame, and kept in position by a 
spring fastened to the door. When bring- 
ing the object into focus, the front slide i» 
removed, and a piece of ground glass is 
placed in the plate-holder, and the lens is 
moved forward and backward till a sharp 
picture is obtained. Cameras of better con- 
