The Birmingham age-herald. [volume] (Birmingham, Ala.) 1902-1950, 1910-07-17
YOUR AUTOMOBILE
HEADLIGHT LENSE
Reflectors Not Ground From
Solid Blocks of Glass
THE NAVY STANDARD
xs Followed by the Manufacturers.
short Focal Lenses Produce In
tense Heat and Crack-Correct
Position for Burner
From the Motor World.

From the Motor World.
For night travelling the headlight is in
dispensable, and though part of the
standard equipment of most every ma
chine now on the market the use or con-
struction of this important adjunct of a
car is but little understood
While a few years since the so-called
French mirror refectors, which We're
nothing more nor less than bent plate
glass, were commonly used for automo-
bile searchlights these have practically
disappeared today except in very cheap
lamps, ground glass refectors being used
in medium and high priced lamps. And
while certain firms still make extrava-
giant claims relative to the mirrors use /
H. lamps of their manufacture, 1t is
fact that the mirror rehectors produced
by the three leading mirror manufac-
turers of today are to all intents and
purposes the same product.
A great many have gained the impress
sion, through statements made to them
by lamp statesmen, that certain mirrors
are ground from solid, square blocks of
glass This is an absolute misstatement
ir so far as automobile lamp mirrors are
concerned, although this method is still
pursued in some higher grades of optical
work The mirrors used in automobile
lamps, however, are all pressed into APP
proximately the finished shape and di-
mansions, and are then ground to exact
standards, the standard, being with the
three concerns above referred to, such
that the focal length of the lens is one
half of its diameter, or, in other words,
that the burner in the lamp having
6-inch mirror should be placed inches
from the center of the lens.
Known as Navy Standard
This is what is known as the navy
standard, and while certain concerns are
advocating a shorter focal length, auto
mobile lamp manufacturers have not, as
n matter of fact, so far produced any
thing which improves upon the standard
set by the United States government.
For while by shortening the focal length
!. is true that the light car be projected
a greater distance, this excessive pro-
lection results in less diffusion Of light
at a nearer range, and for the average
driver the beam projected from navy
standard lenses covers the road satisfac-
torily both in so far as nearby diffu-
sion and length of them are concerned.
There is a further and more serious
fault to be found with short focal lenses,
as the placing of the burner close to the
mirror produces an intense heat which
WA crack the mirror unless the glass of
which it is cast has an excess of lead or
chemical components, in which case the
glass by such addition softens and thus
loses the rigidity which 1t needs to re-
tain its peculiar convex shape Such mir
rors, manufactured of special glass and
rendered comparatively soft SO as to
Withstand the intense heat of the closely
placed mirror, lose, by the resultant dis
tortion, the high and exact refractory
power which they should have.
Ratio Formula Simple
Lenses of any desired focal length can
be manufactured, and the required ratios
for such mirrors arrived at by very, sim
pie calculations, although some Damp
manufacturers are prone to surround the
figures of their own particular formulas
with more or less mystery, and would
lead one to believe that automobile
searchlights are responsible for the dis
covery of optical facts, which in reality
have been matters of record for years.
To secure the results from any gas lamp,
however. the owner would do well to re-
rer as to the correct focal length for his
lamp and see that the lamps are placed
the correct distance from the mirror, as
during shipment, etc, adjusting screws
frequently become loose, burners shifted,
and the purchaser thereupon condemns
lamp which might give him perfect sat
isfa ction were a simple adjustment made
between mirror and burner.