PRESS BULLETIN No. 88.
Florlda Agricullural Experiment St1aon.
USE OF A HAND MAGNIFIER.
BY E. W. BERGER.
An insect pest or other plant disease cannot be successfully controlled
unless one is ready to do a little careful observing for oneself. A good hand
magnifier is indispensable for this, since without its use several stages of insect
life-especially the immature stages (eggs and larvae) of scale insects,
whiteflies and others-must remain more or less unseen and unknown. The
same is true in the diagnosis of plant diseases, such as the withertip of
citrus trees, the different mildews, and other fungus diseases. The useful-
ness of a hand magnifier will also be apparent in determining the condition
of the fungus parasites of insects, such as those of the citrus whitefly,
and those of scale insects. Anyone can use a hand magnifier without
DESIGNATION OF POWERS.
.Manufacturers of hand magnifiers and lenses generally designate them
by the "focus" or "focal distance," rather than by the magnifying or en-
larging power, since the former determines the latter. The "focal distance"
of a lens is a little greater than the distance that the lens should be
held from the object to be magnified, and is usually stated in inches or
fractions of an inch. When we use a hand magnifier as a burning-glass by
holding it in the direct rays of the sun so that the sun's rays are con-
centrated by it on some object placed near it, the center of the bright
disk (image of the sun) formed on the object represents the position of
the focus of the lens. The distance from the center of this "disc" to the
center of the lens is the "focal distance." In catalogs of optical instru-
ments, however, the term "focus" is frequently used instead of "focal dis-
tance." (A hand magnifier should be held very near to the eye in order
to get the best results.) The magnifying power of a lens is generally stated
in diameters of the object magnified. Thus a magnifier of % inch focus
has a magnifying power of about 15 diameters; that is, it will enlarge an
object to about 15 times its actual length and width. Hand magnifiers are
generally made in the following sizes (the "focus," or "focal distance," is
stated first, and the magnification in diameters-written X-follows): 2 inch
focus, X5; 1 inch focus, X10; % inch focus, X15; % inch focus, X20; 1/ inch
focus, X40. It will be noticed that the shorter the focus the greater the
April 11, 1908.
The common hand magnifiers generally sold by jewelers and by dealers
in spectacles serve the purpose fairly well, but are generally not to be com-
pared in quality and magnifying power with the better grade of magnifiers.
A much better magnifier can be obtained by having your local dealer order
one of a certain magnifying power or focus for you from a manufacturer or
extensive dealer in optical instruments, such as Bausch and Lomb Opti-
cal Co., Rochester, N. Y., or Arthur H. Thomas Co., Philadelphia, Pa.
Other manufacturers are: Spencer Lens Co., Buffalo, N. Y.; Queen and Co.,
Philadelphia, Pa. A magnifier of n% inch focus, having a magnifying power
of about 15 diameters, is a very convenient size. The following are the
catalog prices of several standard magnifiers of % inch focus, mounted in
a folding pocket case (the style of mount should be specified when order-
ing):-Doublet $1.00, Coddington $1.00 and $1.25, Triple Aplanat $3.25,
Hastings Aplanatic Triplet $6.25. Magnifiers of different focus differ but
little in price. Of the magnifiers mentioned, the Doublet and Coddington
will probably be the choice of the majority. The Triple Aplanat is
quite superior, however, and should be purchased by persons who have to
work often with a lens, because the cheaper forms may have a bad effect
on the eyes. The Hastings Aplanatic Triplet is recommended only to those
who can well afford to pay the difference in price.
State papers please copy.