Manual

18
concave lens. The procedure is to focus the rays from the source by means of the lens, O
1
, and to locate
the image, F. The concave lens is then placed between O
1
and the mirror and moved along the axis
until an image of S
1
is thrown back as close as possible to S
1
itself. For this purpose it is convenient to
place a source of light behind a small hole on a white screen, and to use the hole as a source so that the
image may appear on the screen close beside the hole. The mirror, M, may be placed in contact with
the concave lens and they may be held together by means of an elastic band. From the diagram, clearly
the focal length of the concave lens must be less than the distance, O
1
F. Thus the return of rays in this
way cannot be obtained for every position of the convex lens with respect to the source. It is a matter
for trial in the first instance, to ensure that the image distance, O
1
F, is sufficiently great and the object
distance, O
1
S
1
, should be made a little greater than the focal length of the convex lens in order that the
image may be thrown on a screen placed at a sufficient distance to the right of O
1
. The concave lens
and mirror should then be moved from this screen towards O
1
until an image appears on the screen at
S
1
. This will then give the magnitude of the focal length required, and a series of measurements may be
obtained by variations of the distance, O
1
S
1
, within certain limits.
P
’P
M
S
1
O
1
O
2
Fig. 20
Fig. 21
F
M