The morning news. [volume] (Savannah, Ga.) 1887-1900, 1899-03-24
OPERA AND FIELD GLASSES.
Good and Rad Qualities of an Ar
ticIe seldom Understood.

From the Optician.
There are few articles sold by the opti-
clan so imperfecty understood as opera,
field and marine glasses. How much
greater is the ignorance of the buyer of
such articles is shown by the enormous
numbers bought from the cheap auction
eer under the glare of gas light and with
regard merely for the snowy coverings.
1t is not beyond the power of the intelii-
gent optician TO counterfeit this TO some ex-
tent, and with the cultivation of knowl-
edge fostered by the Spectacle Makers
Company and the British Optical Asso-
citation surely the study of instruments like
the everyday opera glass can be made
proportionatey remunerative. The pur-
chaser III search of field glass is mucn
more difI1cult to please over the counter
than in an auction room-he frequently
wants an impossible glass, The seller Of
tentimes knows this, yet is quite unable
to explain satisfactorily why such glass
is unobtalnable Again the man in the
street is a poor judge of these articles,
is totally ignorant of any relation between
light and magnifying power He needs
explanation of these things, and the seller
suage and terms such elementary matters
as to enable the buyer 70 see that his con-
fidence may be safely placed in the opti-
clan.
The popularity of the instruments under
consideration is that concaVe lenses are
used as eye pieces lin front Of convex
lenses for magnifying purposes, the pos
sibihty of which was discovered by Medus
about 1900 A. d. Calico seems 10 have
III 4 zen trtl n
any result, making a small telescope hav-
ing a convex lens at one end and con-
cave lens at the other, with which he dis
cerned the satellites of Jupiter and the
spots on the sun. The present-day field
glass is merely an elaboration Of this IN
strument, two tubes being Joined together,
one for each eye. the introduction Of achro-
matte lenses marking the great improve
ment in its construction
The lenses of opera, field and marine
glasses Whatever their number, are al
WAYS grouped az both ends Of the tube
those nearer the eye being known as the
ocular eyepiece, while those at the other
and are called collectively the objective
The action of objective and eyepiece is very
simple An image Of "A far off body such
as the moon, is formed by the convex OBJ
iective at a distance from it approximately
eoua} TO its focal length. That Is 10 say
if a field glass objective of 8-inch focus is
used as a Photographic lens, the inverted
image will be distinct when the ground
glass Of the camera is approximately .
inches away. The ocular or eyepiece is
nlaced in The tube of the field glass be-
tween the objective and its inverted image
AZ distance from the latter approximate
Nv equal to its own focal length, so that If
the concave eyepiece is 2-inch focus and
the above objective Is used, vision will be
distinct when they are about inches
aDG".
A question constantly asked by the buy
or IS "How far will it carry?"-a query
dif5cult to answer on account Of its indefi-
niteness The answers, five, ten fifteen
or twenty miles, may be equally correct, as
everything depends upon the size of the
object viewed, for 1t 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
diameters or number Of times linear meas
urement definite magnifying power is in
dicated. The illumination of the field of
vision must now be considered. II is i'm
,,ossible by any optical arrangement what
ever to obtain an image whose brightest
mart shall surpass the brightest part of the
object Not only so, bui in all instruments
such as we are now considering, where the
image Us enlarged, we bet constantly dc-
creasing brightness, and as the power in
creages the brightness decreases.
This illumination of the field must not be
COMPOUNDED with definition of sharpness.
which depends TO a great extent on the
achromat!sm of the lenses, as win be ex-
nlained later. The uninitiated generally err
IN fhis matter, and constantly prefer lower
newer glasses with a bright field to those
Of higher power and finer definition but
less Illumination. ready method of OBJ
tanning the approximate magnfying pow
er is'io use one tube Of the binocular to
one eve while the other views the real OBJ
iect a means whereby the image can be
superposed on the reality and the enlarge
rnent estimated. A straight course Of
brickwork may be used for this purpose,
the image Of a single brick being OBJ
served to cover three, four or five, as the
case may be.
To compare two glasses is more difficult,
and some practice Is necessary before two