The evening times., 1899-03-25
OPERA AND FIELD GLASSES.
Good and Bad Qualities of Arr
ticIe Seldom UnderstoOI.
fErnm the OnticianN

1cIc stfdom
(From the Optician.)
There are few articles sold by the OPP
tieian SO imperfectiy 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 auctioneer
under the glare of gaslight and with regard
merely for the shOwy coverings II is not
beyond the power OF the intelligent op
tician TO counteract this TO some extent,
and with the cultivation of knowledge fos-
tered by the Spectacle Makers Company
and the British Optical Association surely
the study of instruments Ike the everyday
opera glass can be made proport onately
remunerative. The purchaser in search OF
field glass is much more difficult 10 please
over the counter than in an auction room
he frequently wants an Impossible glass.
The seller oftentimes knows IN 5, yet Is
quite unable to explain satisfactory y why
such a glass is unobtainable. Again, the
man in the street is 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 should be able to expla in simple
language and terms such elementary mat
ters as to enable the buyer to see that his
confidence may be safely placed in the op-
tician.
The peculiarity of the instruments un-
der consideration is that concave lenses
are used as eyepieces in front. OF convex
lenses for magnifying purposes, the pos
sibility of which was discovered by Me-
thus about 1000 a. d. Calico Seems 10
have been the first to utilize the discovery
with any result. making a small telescope
having a convex lens at one end and a
concaye lens at the other, with which he
discerned 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
achromatic lenses marking the great i'm
| provement in its construction.
The lenses of opera. field, and marine
glasses. whatever their number. are always
| grouped at both ends of the tube those
nearer the eye being known as the ocular,
| or eyepiece, while those at the other end
are called collectively the objective. The
| action of objective and eyepiece is very
| simple. An image of far-off body such
| as the moon. is formed by the convex OBJ
| jectiye at distance from it approximate
| ly equal to its focal length. That is to
| say if a field-glass objective of S-inch TO-
Gus is used as Photographic lens. the in
| verted image will be distinct when the
| ground glass of the camera is approximate
| ly s inches away. The ocular or eyepiece
| Is plGCSu III the iuDS Oh the HSlu glass De-
| tween the objective and its inverted image
at distance from the latter approximate
| ly 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 6 inches
apart
a question constantly asked by the buyer
is, How far will il carry "a query diff
cult TO answer on account of its indefinite
l THa
twenty miles, may be equally correct as
everything depends upon the size of the
object viewed, for it is evident that with q
glass of no magnifying power whatever
stars can be seen which are millions OF
miles away. In stating the number of di-
ameters or number OF times linear meas
urement definite magn:fying power is in
dicated, the illumination of the field o
vision must now be considered. 1t is i'm
possible by any optical arrangement what
ever to obtain an image whose brightes
part shall surpass the brightest Part OF tn
object. Not only so, but in al n ument
such as we are now consid ing, where tn,
image is enlarged, We get a coustan " de
creasing brightness, and as tte power in
creases the brightness decrtas s.
This illumination Of the fie d must not b
confounded with definition or SHARPNESS
which depends to a great extent On tn
achromatism of the lenses as vi / be ex
planned later. The unin.tiated generall,
err in this matter, and constant prefe
lower power glasses with bright fed t.
those of higher power and finer definition
but less illumination. A ready rnf tfo1 O
obtaining the approximate magnifyin,
| power is TO use one tube of the b nocua a
to one eye while the other views the rea
object, means whereby the image can b.
superposed on the reality and the enlarge
ment estimated. straight course O
brickwork may be used for this purpose
the image of single brick being observe,
to cover three, four, or five. as the cas-
may be.
To compare two glasses is more difficult
and some practice is necessary before tw
images can be brought together for tn
estimation, but the experiment is vel
worth trying, and the experience is grea
acquisition for judging not only magnify
ing power, but also definition. There is
; constant demand for a high-power glass il
; a shorter form than ii is possible to OBJ
tain. Evidently when there is merely
distance of or 1-2 inches allowed be
tween eyepiece and objective, it is Difficult
) to get anything but low power; for, sup
posing the eyepiece to be 1-2 inch focu
(Which would be a very deep lens). thel
the objective would be about 4 1-2 or 8
inch focal length and the power diane
ter. Suppose the eyepiece were mad
inch the objective would be about inche
and the power merely increased by about
diameter, the number OF rays entering tn
J. eye being greatly diminished, causing
n corresponding loss Of light. =
!, There is a considerable difference in tn
external form of opera, field, and marin
i glasses corresponding to the purposes -fo
j. which they are used. The opera glas
K must be light and convenient for holdin
in one hand when raised to the eye. cor