Evening star., 1899-03-25

cult to answer on account of it's indefinite
ness. The answers, five, ten, fifteen or
twenty miles. may be equally correct, as
everything depends upon the size of the OBJ
Ject viewed, for it is evident that with a
glass of no magnifying power whatever
stars can be seen which are millions of
miles away. In stating the number of di-
aneterS or number of times linear meas
urement definite magnifying power is in
dicated. The illumination of the field of
vision must now be considered. 1t is impos-
sible by any optical arrangement whatever
TO OBTAIN an image whose brightest part
snail surpass the brightest part of the OBJ
Ject. Not only so, but in all instruments
such as we are now considering, where the
IMAGE Is enlarged, we get constantly de-
creasing brightness, and as the power in
creaSeS the brightness decreases
rms illumination of the field must not be
confounded with definition or SHARPNESS
winch depends to a great extent on the ach-
romat1sm of the lenses, as will be explain
ed later. The uninitiated generally err in
TINS matter, and constantly prefer lower
power glasses with a bright field to those
OI hlSher power and finer definition hui less
illumination. 4 ready method of obtaining
the approximate magnifying power is to
use one tube of the binocular To one eve
"hle thc other views the real object a
means whereby the image can be suner
posed on the reality and the enla lareeman,
estimated. A straight course of brickwork
may be used for this purpose the image of
d SHlsie prlck being observed to cover
To compare two glasses is more difficult
ana some practice is necessary before two
images can be brought together for the es-
tlmation, but the experiment is we'll worth
trying and the experience is great ac-
QUESTION for judging not only magnifying
power, but also definition. There is a con-
slant demand for a hgh-pOwer glass in a
shorter form than it is possible to obtain.
Evidently when there is merely distance
of or 5l2 inches allowed between eyepiece
and objective it is difficult to get anything
but low power: for, supposing the eye
piece to be 112-inch focus (Which would be
a very deep lens), then the objective would
be about AZ or 5-inch focal length and the
power diameter. Suppose the eyepiece
were made l inch, the objective would be
about 4 inches and the power merely in
creased by about l diameter. the number
OF rays entering the eye being greatly di-
minlshed. causing ID corresponding loss of
light.
There is a considerable difference in the
external form of opera, fed and marine
glasses corresponding to the purposes for
which they are used. The opera glass must
be light and convenient for holding in one
nahd "hen raised to the eye, consequently
IL should be small AS it is intended for
use chiefly inside buildings and at comDara-
lively short distances no great power is
necessary, ~ or diameters being suffi-
cient, especially as a bright field iS do
slderatum The field glass is required for
greater distances, a higher magnifyiur
power being demanded by the sportsman or
tourist who carries it. The objective is not
necessarily large, as comparatively limited
areas are viewed
The marine glass, as its name indicates.
is designed for use at sea. The requisites
are wide field of vision and as high a
power as IS consistent with good illumi-
nation. This latter consideration will not
allow so many diameters to be obtained
as with the best field glasses They are
generally constructed of low form. with
wide eyepieces and large object glasses
the entire instrument being strongly fitted
with stout tubes and shades for seafaring
purposes. e