Year: 1888

page_text: We, FERDINAND HURTER of Wilmere House, Bold, Widnes in the county of Lancaster,
Analytical Chemist, and VERO CHARLES DRIFFIELD of Mount Pleasant, Appleton,
Widnes in the county of Lancaster, Engineer do hereby declare the nature of this
invention and in what manner the same is to be performed, to be particularly described
and ascertained in and by the following statement :-
Our improvements consist in so arranging logarithmic scales as to enable a
photographer to ascertain at a glance, and with a considerable degree of certainty, the
length of time during which the sensitive plate must be exposed in the camera in
order to procure a satisfactory negative.
Four factors enter into any such calculation: 1st. the time of exposure, 2ndly, the
intensity of the light, 3rdly. the focal length, aperture, and construction of the lens,
and 4thly. the sensitiveness to light of the photographic plate.
We therefore employ four logarithmic scales corresponding respectively to the four
factors mentioned above, time, light, lens and plate.
Two of these scales are attached to a movable slide working between the other two
scales which are fixed.
Upon one of the fixed pair of scales we mark the intensity of the light expressed
in arbitrary degrees, and upon the other we mark what we call and hereinafter define
as the "inertia "or slowness of the plate.
Upon one of the movable pair of scales we mark the various speeds of lenses in a
manner which will hereafter be explained, and upon the other, the time of exposure
in seconds..
Either of these pairs of scales may be fixed and the other pair movable, but owing
to the peculiar construction of the light scale, we prefer to have the light and the
"inertia" scales fixed, and the lens and exposure scales movable.
In order to avoid the use of an instrument for directly measuring the light at the
moment of exposing the sensitive plate we employ as a light scale a diagram shewing
the average relative intensity of the light for every hour throughout the year for any
particular latitude. This diagram consists of a number of curves, each of which
indicates the variation of the light for any given hour of the day throughout the
year. It is necessary to construct separate diagrams for various latitudes.

page_text: In order to construct these diagrams we proceed as follows.
We have ascertained by daily observations extending over several years that the
mean intensity of light at any given hour in the day is about one half of the highest
intensity possible at that hour. We have further ascertained that at or immediately
after sunset or before sunrise the photographic value of the twilight on clear days is
uniform throughout the year, and we have ascertained the numerical value of the
twilight expressed in arbitrary degrees.
We therefore calculate first the highest intensity of the light for every hour of the
day for a certain number of days, say two in each month of the year, for a given
latitude, on the basis that the difference between the intensity of light at 12 a.m. and
at sunset is 100 units in places where the sun culminates in the zenith. We perform
this calculation by multiplying the sine, of the altitude of the sun at the particular
hour by 100. The values thus found for the intensity of light would lead to absolute
darkness at the time of sunrise and sunset. We therefore add to each of these
numbers the value of the twilight which we have ascertained to be equal to about
5 units on this arbitrary scale, the sum we divide by two, and the number so obtained
we term the mean light for that hour.
Having calculated in this way the value of the mean light for a particular hour, say
10 a.m., for two days in each month, we proceed to construct the logarithmic curve
for that hour. We find in a table of logarithms, the logarithm of the numbers, we
use the logarithmic values as abscissæ, and the dates as ordinates and draw through
the points thus found a curve connecting them. This curve represents the variation
of the mean intensity of light at 10 a.m. throughout the year.
In order to still better illustrate the mode of constructing these curves we have
annexed a sheet of drawings. Fig. 1 shews the curve for 10 a.m. for the
latitude 53° 7'.
We mark on the vertical line A B the 182 days from December 21st to June 21st.
We find by calculation as indicated above that 100 times the sine of the altitude of
the sun at 10 a.m. on December 21st is 158 to which we add 520-8. Half of
this number, viz. 10.4 is what we call the mean light for 10 a.m. on December 21st.
The common logarithm of 104 is 1017: Representing graphically the logarithm
of 10 by a length of 100 millimeters, we mark off on a horizontal line drawn through
the point corresponding to the 21st December 101.7 millimeters, and mark that point
as 10a.m. Proceeding in like manner for two days in each month, the other points
of the curve are obtained, and the curve is drawn through them.
In like manner we proceed to obtain the curves for every other hour of the day
that the light is photographically active. We calculate the hour at which light ceases
to be active as corresponding to the time when the sun is 6° below the horizon. We
draw all the curves belonging to one latitude upon one diagram, as shewn in Fig. 2,
and mark them with the hours to which they correspond. On the margins we
indicate the corresponding dates. Fig. 2 represents the diagram for places situated
on the equator.
Owing to the fact that the intensity of the light of the afternoon hours corresponds
exactly to the intensity of the light of the morning hours of the same day, each curve
is marked with two figures, the ordinary figure shewing the morning hour, the
Roman figure the corresponding afternoon hour.
And owing to the fact that to each day between the 21st December and the
21st June there is a corresponding day between the 21st June and 21st December on
which the intensities of the light are the same, it is not necessary to mark the curves
for the whole of the year: it suffices to calculate and mark them for half the year;
and to place on the opposite side another scale of dates returning from the 22nd June
to the 21st December in contrary direction as shewn in Figs. 1 & 2.
As already stated we provide a number of such diagrams for the different latitudes.
On the second fixed scale we mark what we have previously spoken of as the
" inertia " or slowness of the sensitive plate. By this term we express the time,
measured in seconds, which the sensitive plate must be exposed to direct diffuse
daylight, of the intensity 1° on our arbitrary scale, in order that, on development,

page_text: sufficient of the silver salt on the plate may be reduced to silver, so as to become as
dense as the sky of a well exposed landscape negative ought to be. It will be
perceived, that the slower the plate the greater will be the time necessary for
obtaining this result.
As the "inertia" of the plates of different manufactures vary, we provido a whole
scale of "inertias" ranging from 0.01 of a second to 1 second, but we may state at
once that most of the gelatino-bromide plates in the market have, according to our
experiments, inertias" ranging from about 0·08 to 0.35 of a second, but those
generally known as "ordinary," "rapid" and "extra rapid" have "inertias" of
about 0.2, 0.14 and 0.08 respectively.
"
We have ascertained these "inertias" of the plates by means of accurate measure-
ments of the light at the moment of exposure, but we shall hereafter show how,
by means of our present invention, every operator can ascertain the "inertia" of any
plates he may desire.
In constructing this "inertia" scale, we simply take the logarithms of the numbers
as abscissæ and mark the points with the natural numbers as is well understood in
the construction of ordinary slide rules. Care must be taken that the same unit be
chosen for the logarithmic value of 10 in this scale as in the light scales. Fig. 3
shews the arrangement and special marking of the "inertia" scale.
:
In consequence of the various "inertias" of different plates, wo prefer to apply a
complete inertia" scale as described, but it is obvious that in the case of plates of
known "inertia," a single line would suffice, or a movable index might be
substituted for it.
The first of the movable pair of scales is what we call the Lens scale. It is shewn
in Fig. 4. It consists simply of a number of lines in groups of three, marked
respectively 1, 2 and 3, and below each group with the usual mode of expressing the
value of the aperture of the lens as a fraction of its focal length, which in our example
is the system adopted by the Photographic Society of Great Britain, but any other
system can be easily adapted. Thus the group of lines marked belongs to lenses
the apertures of which have been stopped down so that the optical value of the
diameter of the aperture is 1 of the focal length.
-
=
As is well known, the intensity of the light in the image produced by such a lens,
if the lens were perfectly transparent and reflected no light from its surfaces, would
be (6) of the intensity of the light sent out by the object, and it would
consequently require, with this lens (16) 256 times the exposure which would be
necessary if light of the same intensity as the object sends out had acted upon tho
plate without the interposition of a lens. We find however by experiment that this
is not correct. Owing chiefly to reflection from the surface of the lens more time
is requisite, and more time in proportion to the number of reflecting surfaces which
the lens has. We find that the time is about 15 per cent. more in the case of single
lenses, 32 per cent. more in case of double combinations, and 53 per cent. more in case
of triple combinations. For this reason we have three lines for each ratio of aperture
to Focal length.
The lines marked 1 refer to single lenses, the lines 2 to double combinations and
the lines marked 3 to triple combinations.
The exact place where each line has to be marked upon the scale is found as
follows.
We square the denominator of the fraction indicating the ratio of aperture to
focal length. We multiply this by 115 for single lenses, by 1.32 for double com-
binations and by 1.53 for triple combination lenses of the products thus obtained we
take the logarithms; considering the logarithm of 10 expressed by 100 millimeters,
we mark off the corresponding logarithms, and at the respective lines we write the
figures 1, 2 and 3, and under them the ratio of aperture to focal length. As there
are no lenses which yield figures smaller than 10, our scale begins with 10 and
proceeds to 10,000; the spaces between 10 and 100, 100 and 1,000 1,000 and 10,000
being all 100 millimeters cach.

page_text: For example it is required to find the place at which to mark for a double
combination.
The square of 32 is 1024, this figure multiplied by 1.32 for reasons above stated
gives 1351. The logarithm of 1351 is 3.1306. If our Lens scale started with 1,
we should have to mark at 313 millimeters the place for (2). But as the scale
starts with 10 for reasons already stated, the place will have to be marked at
100 millimeters less, i.e. at 213 millimeters. In exactly the same way we proceed to
mark the ordinary ratios in use as shewn on Fig. 4.
The last scale, the time scale is simply an ordinary logarithmic scale of the natural
numbers as used in ordinary slide rules. It expresses the time of exposure in seconds,
beginning with 0.1 of a second and proceeding to 100 seconds. The scale shewn
in Fig. 4. It, together with the Lens scale, forms the movable slide.
It will be readily understood that any other unit instead of millimeters may be
adopted for the construction of these logarithmic scales, but the same unit must be
adopted for all of them.
The four scales which we have now described in detail may be combined into one
instrument in various ways, two of which are shewn in Figs. 5 and 6.
Fig. 6 represents a sectional elevation of the form we prefer to adopt when using
several light diagrams for different latitudes.
A is a block of wood perforated with an oblique slot through which the diagram B,
mounted on card or other stiff material, slides up and down. In close proximity to
this diagram is the sliding strip C bearing the lens and time scales, and fixed on
the block in juxtaposition to the sliding scale is the "inertia" scale D, the initial
point of which must be exactly opposite the initial point of the light scale on the
diagram.
"
Fig. shews a form of instrument which we prefer when restricting ourselves
to one latitude only. In this case we wrap the paper diagram round a roller E,
in such manner that the dates become successively visible on turning the roller.
The other scales are fixed as shewn in the other form, the lens scale being as
close as possible to the cylindrical light scale.
To ascertain by means of this instrument the "inertia" of any given plate, we
assume the plate in the first instance to be of medium sensitiveness, to which
corresponds the "inertia" 0.14.
Supposing the experiment to be made on the 25th of May at 9 a.m. or 3 p.m., we
take an ordinary landscape say with a doublet lens stopped to. We set the
diagram till this date corresponds with the edge of the lens scale, and move the lens
scale until (2) is exactly opposite the curve marked 9 and III.
Opposite the assumed "inertia" 0.14 on the "inertia" scale we find an exposure
indicated on the time scale of 5 seconds. If we believe the light to be a mean
light we expose for five seconds. If upon development this exposure yields a
satisfactory negative, the assumed "inertia" is correct. On the other hand, if
the negativé is under or over exposed, the "inertia " was respectively too small
or too large, and a second experiment, with another "inertia" is desirable, and
will generally decide the question.
Having ascertained the "inertia" by such experiments, it is afterwards easy to
find the exposure necessary at any other time and with any other lens. Suppose for
instance a landscape had to be taken on the 10th of January at 12 noon with a single
lens and with a plate of which the "inertia" is 0.2, we set the light diagram to
the date as before, and move the point (1) on the lens scale opposite the curve 12
on the diagram, and find opposite the "inertia " 0.2 an exposure of 15 seconds
indicated.
The method of using the cylindrical diagram will be obvious after these explanations.
The exposures thus indicated by our instrument are strictly correct only for mean
light and ordinary landscape work.
Of course when taking pictures differing materially in their illumination from
ordinary landscapes, suitable allowances must be made -

page_text: Instead of employing one of our light diagrams, the photographer may use an
instrument for directly measuring the light at the time of exposure. In this case the
light diagram must be replaced by an ordinary logarithmic scale of natural numbers
similar to our "inertia" scale, but marked from 1 to 100, and instead of setting the
lens scale to the particular day and hour, as in the case of our light diagrams, it is set
to the figure indicated by the instrument for measuring the light, the "inertia" of
the plate being found as previously explained.
Having now particularly described and ascertained the nature of our said inven-
tion and in what manner the same is to be performed, we declare. that what we
claim is :-
1. An instrument for calculating photographic exposures consisting of a com-
bination of specially marked logarithmic scales substantially as hereinbefore described.
2. The application of several light scales for different latitudes to one instrument,
substantially as hereinbefore described.
FERDINAND HURTER.
VERO C. DRIFFIELD.