THE YOUNG 
struction, with bellows and focussing screw, 
are provided with a special frame holding 
the ground glass. The purchaser should 
filways convince himself, by actual measure- 
ment, that the ground glass and the sensi- 
tized plate in the plate-holder occupy ex- 
actly the same plane. The construction of 
a photographic camera requires great skill 
and numerous tools, and therefore it might 
be advisable for the amateur to buy a ready 
made camera, with plate-holder. 
Of simple lenses the plano-convex lens, 
with plane side towards the object, gives, 
after the meniscus with the concave side to 
the object, the most correct image. For 
landscape photography, simple lenses will 
do very well, lenses of larger diameter being 
preferable to those of a short diameter. 
For portraiture a combination lens, or por- 
trait lens, is indispensable, though good 
results may also be obtained with a simple 
lens of large diameter. 
The use of a simple lens always necessi- 
tates a small diaphragm or stop, in order to 
counteract spherical aberration and the 
want of achromaticity. Stops can easily be 
made of pasteboard to fit exactly into the 
tube of the lens. Fig. 5 represents a dia- 
phragm. The projections, a, h, facilitate 
removing the stop. The centre of the dia- 
phragm must coincide with the centre of 
Fig. 5. 
the lens. The position of the diaphragm, 
either in front or behind the lens, is imma- 
terial, though a slight difference is thereby 
produced. Small diaphragms, that is, dia- 
phragms with a small aperture (1-4 to 1-12 
inch), increase considerably the depth of 
the focus, near and distant objects will be 
in focus at the same time, and the picture 
on the ground glass will be everywhere 
SCIENTIST. 49 
equally sharp. The disadvantage of a very 
small opening in the diaphragm lies in the 
fact that it diminishes the illumination of 
the image on the ground glass, and thereby 
renders a longer exposure necessary. 
In order to ascertain whether the actinic 
and visual foci in a lens coincide, a thin 
piece of violet colored glass is placed before 
the lens, and the object is focussed on the 
ground glass. The violet colored glass is 
now removed, and if the image still appears 
sharp, the actinic and visual foci coincide. 
The visual focus is that distance between 
the lens and the ground glass which pro- 
duces on the latter a sharp picture visible 
to the eye. A picture in the actinic focus 
may not appear sharp to the eye, and yet 
the picture will require a shorter exposure, 
and the negative may exhibit the most dis- 
tinct outlines. The violet colored glass can 
just as conveniently be fixed permanently 
in the tube in front of the lens. For focus- 
ing, a piece of black velvet, or some other 
material impenetrable to light, about one 
yard square, is usually employed. 
For copying pictures, the camera can be 
placed on a common table; for landscape 
photography and portraiture we need a tri- 
pod, which is easily constructed. A piece 
Fig. 6. 
of oblong board, a little larger than the 
camera, is provided with three good hinges, 
one hinge in front, and one on each side, 
as shown in Fig. 6. Each hinge is fastened 
to a narrow slat of wood, which serves as 
leg. On the top of the oblong board you 
may nail some strips of wood, at such a dis- 
tance apart that the camera can be tightly 
squeezed into them. 
