The Physiological Light Unit.
DIFFUSED LIGHT.
The physiological values of light intensities determined by the
absorptive method, differ in some respects, from the intensities based
on the inverse ratio of the squares of distance between the shadows of
two lights.
However valuable this method of calculating light intensities may
be from a mathematical point of view, it does not express the
physiological appreciation of light differences.
The dimensions of the light unit used in the following experiments have
already been described.
This method cannot deal exhaustively with intense direct lights, on
account of the presence (activity) of the disturbing red ray which
prevents total absorption. Such lights must first be modified by
intercepting media of known diffusive value, or by reflection from a
white surface.
One object of these experiments was to obtain more insight into the
physiological conditions of light, as bearing on the question of
standardization.
The first two experiments are records of intensity changes by time,
in Morning and Evening light, and are of interest, as bearing on the
lowest luminosities for reading, for viewing objects at different
distances, and for defining the limits at which colours visually
disappear. The measurements are marked in neutral tint units and
plotted in curves in Charts 1 and 2.
The numbers on the single curve in the Morning chart represent total
absorption of the direct light 20 degrees above the horizon. The upper
curve in the Evening chart also represents the direct light, whilst
the under curve represents the light values as reflected from a lime
sulphate surface; except in the case of the reading notes when it
represents the printed paper surface. The difference between the two
curves is the loss of light incident on reflection, but this must not
be rigidly interpreted for all cases, as there is reason for supposing
it varies with different lights.
DIRECT LIGHTS.
In measuring the physiological intensity of direct lights, the presence
(activity) of the unabsorbable red ray, prevents their being dealt
with directly by the absorptive method. Such lights can, however, be
made measurable by a sufficient diffusion, as already explained in the
case of direct sunlight, the proportion of diffusion required, being
more or less according to the intensity of the light; in the following
examples, one reflection from a white surface or from the ordinary
grease spot arrangement, was found to be sufficient.
Note.--Light reflected from grease is not above suspicion, it
being governed by the law of specific absorption already dealt
with.
[Illustration: Chart 1.
Evening Light March 18^{th}. 1906.
_Upper Curve 20° above Horizon
Lower Curve as Reflected from a
1 Foot Square Lime Sulphate Screen_
]
[Illustration: Chart 2.
Morning Light Dec^r. 17^{th}. 1907.
_20° above Horizon_]
[Illustration: Chart 3.
Luminous Intensities of _1, 2 & 3 Standard Candles_.]
[Illustration: Chart 4.
Luminosity of Gas from Burner
_Reduced to Equal 1, 2 & 3 Candles at 1 Foot._]
[Illustration: Chart 5.
Luminosities of Paraffin Lamp
_Reduced to 1, 2 & 3 Candles at 1 Foot._]
The experiments were carried out on a home-made twelve-foot photometer,
with the usual protected lantern at each end, one being removed for
present purposes. The grease spot carrier was replaced by a one-foot
square diaphragm, with a standard white surface; this travelled the
whole length of the photometer at right angles to the light, and the
readings made at one-foot intervals at an angle of 45 degrees, 10
inches distance from the white surface.
Charts 3, 4 and 5 deal with experiments, with one, two and three
standard candles of the London Gas Referees. The curves represent
the physiological rate of declining luminosities by distance. Some
characteristic differences from other lights are brought together in
Charts 4 and 5. The slight irregularities in the curves are probably
due to the readings being made at half unit intervals. These acting
sometimes in opposite directions, fully account for want of symmetry in
the curves.
A noticeable feature in these experiments is the small amount of
physiological luminosity added by each successive candle to the
first. Theoretically, if one candle equals 21 units intensity, two
should equal 42, and three 63, whereas the physiological additions of
luminosity are not only much less, but vary with different luminous
intensities, as will be seen by the following comparisons:--
1 2 3
Candle Candles Candles Total.
Theoretical 21 + 21 + 21 = 63
Physiological 21 + 3 + 6 = 30
Chart 4 represents the Luminosity of gas from a batswing burner
reduced to one, two and three candle power at one foot distance.
1 2 3
Candle Candles Candles Total.
Theoretical 21 + 21 + 21 = 63
Physiological 21 + 3 + 7 = 31
Chart 5 represents the Luminosity of a hand Paraffin Lamp similarly
reduced.
1 2 3
Candle Candles Candles Total.
Theoretical 21 + 21 + 21 = 63
Physiological 21 + 3 + 6 = 30
PHOTOGRAPHIC ENERGIES OF DIFFERENT RAYS
The following experiments are preliminary only, and were undertaken to
determine the relationship of the several colours of the tintometrical
scales, to their associated photographic values, under different
conditions of light and times of exposure, the end in view being the
hope of standardizing screens, papers and impressions.
The standardized colours act as selective light absorbents on the same
principle as screens for trichromatic colour work, but differ from
these in their having a definite standard of colour depth and colour
purity.
Work of this character requires paper of a known degree of
sensitiveness, but on inquiry it was found that no reliable standard
had as yet been established. On this, six makes of “white” paper were
purchased in the open market and submitted to exposures under the
following conditions:--
Six slips of the sensitized papers were covered by a thin metal plate,
pierced by six rows of apertures, a complete row lying over each
sample of paper. The rows contained seven apertures each, one being
left uncovered to receive the full energy of the impinging light. The
remaining six were covered respectively by a Red, Orange, Yellow,
Green, Blue or Violet standardized screen of 15 units colour intensity;
the whole was fitted into a suitable exposure frame. Many exposures
were made, from which the four following were selected:--
No. 1. 20 min. exposure to a dull sky.
" 2. 10 " " " bright sunlight.
" 3. 20 " " " "
" 4. 30 " " " "
The results are arranged in Tables VI and VII.
Table VII contains results of different exposures under sunlight
conditions.
TABLE VI.
DULL SOUTH LIGHT.
20 Minutes’ Exposure on White Sensitized Papers.
-----+-----------------+-----------------+------------------
Pap-| | |
ers | OPEN SCREEN. | RED SCREEN. | ORANGE SCREEN.
| | |
-----+-----------------+-----------------+------------------
| Blk. Or. Red. | Blk. Or. Yel. | Blk. Or. Red.
1 | 7·8 4·2 6·0 | 2·0 4·8 3·0 | ---- ·3 ·2
| | Red. |
2 | 6·0 5·0 5·5 | 1·9 3·1 ·4 | ---- ---- ----
| | Yel. |
3 | 7·8 3·2 7·0 | 2·7 4·7 3·0 | ---- ---- ----
| | Red. |
4 | 7·2 4·3 6·0 | 2·2 3·2 ·2 | ---- ---- ----
5 | 5·8 7·2 4·0 | 1·9 2·3 ·8 | ---- ---- ----
6 | 5·6 7·4 4·0 | 1·8 2·6 6 | ---- ---- ----
-----+-----------------+-----------------+------------------
-----+-----------------+-----------------+-----------------+------------------
Pap-| | | |
ers | YELLOW SCREEN. | GREEN SCREEN. | BLUE SCREEN. | VIOLET SCREEN.
| | | |
-----+-----------------+-----------------+-----------------+------------------
| Blk. Or. Red. | Blk. Or. Red. | Blk. Or. Red. | Blk. Or. Red.
1 | 2·0 3·0 1·6 | ---- ---- ---- | 3·9 7·6 3·5 | ·2 ·4 1·0
| | | Yel. |
2 | ·4 ·4 1·0 | ---- ---- ---- | 4·3 9·2 4·5 | ·2 ·8 ·8
| | | Red. |
3 | 1·2 ·5 ·8 | ---- ---- ---- | 3·7 9·3 2·0 | ·7 ·6 ·9
| | | Yel. |
4 | 1·2 ·1 1·2 | ---- ---- ---- | 3·4 1·1 1·0 | ·7 ·7 ·9
5 | 1·0 ·3 ·3 | ---- ---- ---- | 3·3 8·7 2·0 | ·3 ·3 1·2
| | | Red. |
6 | ·2 ·3 ·4 | ---- ---- ---- | 3·6 8·9 1·5 | ·4 ·2 ·9
-----+-----------------+-----------------+-----------------+------------------
TABLE VII.
BRIGHT SUNLIGHT (South), 18--9--12.
10 Minutes’ Exposure on White Sensitized Papers.
---+-------------+---- --------+--------------+---------------
NO.| OPEN SCREEN.| RED SCREEN. |ORANGE SCREEN.|YELLOW SCREEN.
---+-------------+-------------+--------------+---------------
|Blk. Or. Red.|Blk. Or. Yel.|Blk. Or. Red.|Blk. Or. Red.
1 | 8·4 1·6 6·5|2·8 6·8 1·9| ·34 1·41 ·55| 2·3 7·1 --
2 | 8·2 2·0 6·3|2·7 4·7 3·6|-- -- -- | ·8 1·9 ·70
3 | 7·8 2·2 7·5|2·4 7·0 4·1|-- -- -- | 2·4 4·2 ·80
4 | 7·8 1·6 8·6|2·4 5·4 3·2|-- -- -- | 1·5 2·3 ·70
| | | |
5 | 6·4 4·4 7·7|2·4 5·2 3·0|-- -- -- | ·9 1·8 ·80
6 | 5·2 6·3 5·5|2·3 5·1 3·0|-- -- -- | 1·1 2·0 ·80
BRIGHT SUNLIGHT (South), 18--9--12.
20 Minutes.
| Vio. Red.| Or. Red.| Or. Yel.| Red.
1 |10·6 1·4 5·0|7·0 2·2 8·3|1·1 3·6 ·9 |10·0 -- 7·0
| | | Red.| Or.
2 |10·0 -- ·8|7·0 2·6 7·4|-- 1·0 ·3 | 7·4 2·0 3·6
| Or. | | |
3 | 9·2 1·6 2·2|4·5 5·5 8·0|1·8 7·4 -- | 5·2 4·2 8·1
| Or. | | |
4 |10·6 ·4 3·5|6·0 4·2 8·3| ·44 1·36 ·8 | 9·0 2·5 5·0
5 |10·0 ·8 5·7|6·2 4·0 8·3|-- ·7 ·45| 7·0 5·0 4·0
| | | |
6 | 6·2 4·6 7·7|5·0 7·5 4·0| ·2 ·4 ·5 | 5·4 7·1 4·5
BRIGHT SUNLIGHT (South), 18--9--12.
30 Minutes.
| Or. Red.| Or. Red.| Or. Red.| Or. Red.
1 |10·6 ·9 1·5|6·4 4·4 5·7|2·6 7·8 1·1 | 5·2 3·4 10·4
2 | 9·2 1·4 2·9|6·2 5·3 5·5| ·1 ·7 ·4 | 7·8 3·0 5·2
3 |10·8 1·2 1·5|7·2 3·4 8·4| ·7 2·0 ·2 | 9·2 1·6 5·2
4 |10·0 2·0 1·5|6·2 4·4 6·9| ·5 1·0 ·7 | 8·2 2·0 7·3
| | | |
5 | 9·6 4·4 4·5|5·8 5·0 7·2| ·2 1·9 ·7 | 6·2 3·8 6·5
6 | 6·6 7·9 5·0|4·8 7·2 5·0| ·2 ·5 ·55| 5·0 7·5 2·0
---+-------------+-------------+--------------+---------------
BRIGHT SUNLIGHT (South), 18--9--12.
10 Minutes’ Exposure on White Sensitized Papers.
---+-------------+-------------+---------------
NO.|GREEN SCREEN.| BLUE SCREEN.|VIOLET SCREEN.
---+-------------+-------------+---------------
|Blk. Or. Red.|Blk. Or. Red.|Blk. Or. Yel.
1 | ·26 ·69 ·10|5·2 4·4 8·4 |1·75 3·65 ·40
2 |-- ·15 ·03|6·0 3·8 6·7 |1·7 3·9 ·40
3 | ·22 ·02 ·04|5·6 5·9 6·5 |1·5 3·5 1·2
4 | ·19 ·05 ·12|4·8 5·6 6·6 |1·3 3·2 --
| | | Red.
5 |-- -- -- |4·0 10·5 2·0 | ·6 1·7 1·0
6 |-- -- -- |4·0 11·5 1·0 | ·5 1·9 1·1
BRIGHT SUNLIGHT (South), 18--9--12.
20 Minutes.
| Or. Yel.| Or. Red.| Or. Red.
1 |3·1 6·1 1·8 |8·0 2·0 8·5 |4·5 7·5 4·5
| | Or. |
2 |1·9 3·7 ·8 |8·8 ·6 3·6 |5·0 6·5 4·5
| | |
3 |1·0 7·0 1·6 |8·6 1·4 3·5 |4·0 8·5 4·5
| | |
4 |1·8 4·8 1·2 |8·0 2·2 6·8 |4·5 7·5 4·5
5 |1·7 4·9 1·4 |7·6 2·4 8·0 |4·0 8·5 4·0
| Red.| |
6 |1·8 3·4 ·4 |6·4 4·4 8·2 |3·5 8·5 1·0
BRIGHT SUNLIGHT (South), 18--9--12.
30 Minutes.
| Or. Yel.| Or. Red.| Or. Red.
1 |1·0 6·8 ·2 |6·4 2·6 7·5 |4·2 6·8 6·0
2 |1·8 3·4 ·2 |8·4 ·8 3·8 |5·4 8·1 1·0
3 |2·2 5·2 2·2 |8·0 2·0 7·5 |4·5 8·5 4·0
4 |2·5 4·7 2·0 |8·0 1·6 7·4 |3·7 10·3 1·0
| | | Yel.
5 |1·6 4·2 2·8 |7·6 2·6 6·8 |3·1 9·4 1·5
6 |1·5 3·9 ·2 |6·4 7·6 5·5 |3·4 9·7 1·5
---+-------------+-------------+---------------
It would be unsafe to draw definite conclusions from a few experiments,
but so far as permissible, the results show considerable differences,
both in depth and in colour, of the energy of the different rays, for
instance--
Compare Nos. 1 and 6 under 20 min. sunlight.
Black. Orange. Red.
No. 1 10·6 + 1·4 + 5·0
No. 6 6·2 + 4·6 + 7·7
or again 3 and 6, the maximum and minimum, under 30 min. exposure.
Black. Orange. Red.
No. 3 10·8 + 1·2 + 1·5
No. 6 6·6 + 7·9 + 5·0
The sensitiveness of Nos. 2, 4 and 5 appears to have been exhausted by
20 min. exposure to sunlight, further exposure showing no reaction;
whilst the sensitiveness of Nos. 1, 3 and 6 do not appear to have been
exhausted by 30 min. sunlight exposure.
Other noticeable points are the small action under the Orange, Green
and Violet screens, and the greater, although variable proportion of
colour to black under all the colour screens.
TRICHROMATIC COLOUR SCREENS.
Table VIII contains the colour measurements of five sets of screens for
trichromatic colour work which have come from time to time under the
writer’s notice.
The measurements are the tintometrical colour units required to match
the screens under daylight conditions, and are classified under the
theoretically accepted terms of Red, Green and Violet.
TABLE VIII.
----+-------------------------+-------------------------+--------------------
Set.| RED SCREENS. | GREEN SCREENS. | VIOLET SCREENS.
----+-------------------------+-------------------------+--------------------
No.| Red. Or. Vi. Light. | Yel. Green. Or. Light. | Blue. Green. Black.
1 |119·0 ---- ---- 1·6 | 27·0 36·0 ---- 9·0 | 32·0 1·4 6·6
2 | 24·2 +·8 ---- ---- | 8·9 ---- ·1 ---- | 13·0 ---- 3·1
3 | 26·4 ---- ·8 ---- | 7·5 ---- 1·4 ---- | 16·6 ---- 3·2
4 | 22·6 ---- ---- ---- | 7·6 ---- 1·4 ---- | 15·3 ---- 3·1
5 | 21·0 ---- 9·0 ---- | 28·0 14·0 ---- ---- | 28·0 ---- 2·8
----+-------------------------+---------------------------+--------------------
The Red screens are all practically pure except No. 5, which transmits
also 30 per cent. Violet. No. 1 is distinguished by its greater colour
depth and purity, the degree of which latter is recorded at 1·6 light
units brighter than standard.
In the Green division, only Nos. 1 and 5 transmit any green, No. 1
transmitting also 42·8 per cent. yellow and being 9·0 units brighter
than standards; No. 4 transmits 66·6 per cent. yellow to 33·3 per cent.
green; Nos. 2, 3 and 4 are all yellows tinged with orange.
In the Violet division, Nos. 2, 3, 4 and 5 are all pure blues; No. 1 is
tinged with green.
Appendix I
COLOUR EDUCATION
The time has passed when it might have been considered necessary to
preface a handbook on the teaching of colour by arguments to prove that
it is a legitimate subject of instruction in schools, but it has not
hitherto been sufficiently recognized that the early stages of such
instruction must be on sound lines and that nothing must be taught
which will afterwards have to be unlearned.
Apart from the pleasure its sensations give to all properly constituted
persons, the study of colour has an intellectual value in common with
other branches of science. It strengthens the judgment by constantly
requiring thought and precision in definition, it also develops the
faculty of colour perception even to the point of curing some forms of
colour blindness. In addition to this, it forms a necessary part of
the instruction in all schools in which drawing is properly taught by
methods which demand from the pupils faithful representations of the
appearances of actual objects in colour as they are seen.
In the past the systematic study of colour has been more or less
neglected from two principal causes: first, the want of a comprehensive
scheme of colour nomenclature capable of describing all colours in
terms precise enough for general understanding and record; and, second,
the absence of any reliable means of reproducing any specific colour
if lost or faded. Both these conditions may now be secured by the use
of the standardized coloured glasses supplied with the Colour Educator,
and this work is intended to bring the subject before teachers in such
a way as to make each point perfectly easy of demonstration to a class
in a systematic manner.
To-day the value and importance of a keen perception of colour and of
an apparatus furnishing definite colour standards, though perhaps not
much appreciated by the general public, are widely recognized in the
industrial and scientific world; and it is evident that in these days
of keen commercial competition between nations we cannot afford to
neglect any means which will enable us to maintain present industries
and to develop new ones.
=General Remarks.=--It is not advisable to introduce colour theories to
pupils before they know the names of the different colours, but, as the
glasses used in the apparatus are graded for colour-depth according to
a set of scales now generally accepted as of standard value, a short
description of the derivation of the colour names will be of service
when the pupils are sufficiently advanced.
The names of the six spectrum colours, Red, Orange, Yellow, Green,
Blue, and Violet, are accepted by common consent as describing the
principal hues into which a beam of white light can be resolved by
a diffraction grating or by prismatic refraction. They are also the
colours distinguishable in objects of everyday life, and the following
Educational Method is based on the fact that they can be separated at
will from ordinary daylight. Therefore the first educational step is to
associate these six colour rays with their respective names, the pupils
being made to understand that there are many degrees of depth in each
colour.
=The Applications of Colour to the Work of Everyday Life= are so
universal that a complete list is almost impossible, though some of
the most important are mentioned below. In a general way the visual
characteristics of every visible object are determined by contrasts of
light and colour, outline itself being governed by differences of light
intensity.
In Nature, colour is practically universal. There are few objects
perfectly white. Most of them have colour of greater or less
complexity; even snow under a cloudless sky has a blue tint which
is measurable against such white objects as pure lime sulphate,
zinc white, etc., the blue tint being manifestly reflected from the
cloudless sky, as it disappears under a cloudy overcast.
=Some of the Scientific Applications of Colour.=--It associates colour
sensations with definite names and values; discovers and classifies
cases of colour-blindness, and is a preparation for the physical
study of light. It is also essential for studying the physiological
structure of the organs of vision, for disclosing abnormal conditions
of the blood, and for measuring the colour of the hair and skin for
the anthropological classification of races. It is used in general
laboratory work for analytical and original investigation; and
it furnishes standards of value for the petroleum industry, the
International Tanning Association, the inter-States Cotton Seed Oil
Association, etc. It is also one of the principal factors in all
artistic industries; for, besides having an important educational
value in questions of harmony, contrast, and taste, it is of direct
commercial value in such industries as dyeing, calico printing,
all woollen industries, wall-paper printing, paint making, house
decoration, etc.
=General Instructions for Using the Apparatus.=--The apparatus consists
of a frame having six little windows, fitted with sliding shutters, and
a tray containing eighteen standardized glasses.
The frame must be placed on a table or stand facing the children, with
a window or some other source of diffused white light behind it. Care
must be taken not to have a coloured background of any kind.
The glasses are in three colours, Red, Yellow, and Blue, of different
depths. The depth of colour is marked in figures on each glass, and
corresponding numbers in the three colours are of equal intensity. It
is of importance that the six spectrum colour terms should be the only
ones used in the preliminary stages. The first step in colour teaching
must be to develop and train the perceptive faculties of the children
so as to enable them to express in words the colour sensations which
are excited. For this purpose it is necessary to begin by demonstrating
that the six spectrum colours Red, Orange, Yellow, Green, Blue, and
Violet, are derived from white light.
Red, Yellow, and Blue should first be dealt with, and _for practical
work_ each pupil should be supplied with three water-colour pigments
closely corresponding in hue to the standardized glasses, viz., Crimson
Lake, Lemon Yellow, and Cobalt Blue; also with a piece of white paper
ruled into six small rectangular spaces corresponding to the little
windows of the apparatus.
As each colour is demonstrated by means of the glass in the apparatus,
each pupil should paint the corresponding pigment in its proper place
on the paper.
The little shutters being placed in all the six little windows, remove
the top left-hand shutter and insert a deep _Red_ glass, thus showing
=Red.=--The pupils will name this and then paint the corresponding
colour on their papers.
Next, remove the shutter below the first one, and insert a _Yellow_
glass of the same numerical value as the Red one, thus showing
=Yellow.=--The pupils will name this and then paint the corresponding
colour on their papers.
Now remove the shutter next below and insert a _Blue_ glass of the same
numerical value as the Red and Yellow ones, thus showing
=Blue.=--The pupils will name this and then paint the corresponding
colour on their papers.
+--------+--------+
| 1 | |
| | |
| RED | |
| | |
+--------+--------+
| 2 | |
| | |
| YELLOW | |
| | |
+--------+--------+
|3 | |
| | |
| BLUE | |
| | |
+--------+--------+
There are now exposed to view the three colours which are by artists
commonly called primaries, but it will be found convenient to term Red,
Yellow, and Blue the _Dominant_ colours of this system.
The second step is to show how the three secondary or subordinate
colours are derived or developed.
Remove the top right-hand shutter and insert a deep Red and a deep
Yellow glass of equal depth, showing the pupils that these two colours
combined in equal proportions develop
=Orange.=--The pupils will name this, and will then mix their Red and
Yellow pigments to obtain a similar Orange which will be painted in its
proper place on their papers.
Now remove the shutter next below and insert a deep Yellow and a deep
Blue glass of equal depth, showing the pupils that these two colours
combined in equal proportions develop
=Green.=--The pupils will name this, and will then mix their Blue and
Yellow pigments to obtain a similar Green which will be painted in its
proper place on their papers.
+----------+----------+
| 1 | 4 |
| | |
| RED | ORANGE |
| | |
+----------+----------+
| 2 | 5 |
| | |
| YELLOW | GREEN |
| | |
+----------+----------+
|3 | 6 |
| | |
| BLUE | VIOLET |
| | |
+----------+----------+
Remove the last right-hand shutter and insert a deep Red and a deep
Blue glass of equal depth, showing that these two colours combined in
equal proportions develop
=Violet.=--The pupils will name this, and mix their Blue and Red
pigments to obtain a similar Violet, which will also be painted in its
proper place on their papers.
Now are exposed to view on the left-hand side the three primary or
dominant colours, and on the right-hand side the three secondary
or subordinate colours, and the whole frame is filled with the six
spectrum colours in equal colour depth. Corresponding to the colours
in the frame, each pupil’s paper should show a similar arrangement of
colours, and the pupils can be taught their spectrum order by reading
them in rows horizontally--Red, Orange, Yellow, Green, Blue, and Violet.
The teacher should now take out the coloured glasses and replace the
shutters, except the two top windows, one of which is left open to
show white light, and the other filled with three equally deep glasses
in Red, Yellow, and Blue, showing either black or neutral grey, and
demonstrating the total or partial absorption of light according
to their higher or lower numerical unit value. It is of the utmost
importance to bear in mind that the glasses are graded for diffused
daylight, and that all artificial lights are more or less coloured and
would give a different effect. The same remark applies to light taken
direct from coloured objects.
In this set the six windows are in one horizontal line, and should be
uncovered in the following order:
{ No. 1 Window for Red.
Dominants { " 3 " Yellow.
{ " 5 " Blue.
{ " 2 " Orange.
Subordinates { " 4 " Green.
{ " 6 " Violet.
One advantage in this method is that when all the colours are in they
are arranged in their spectrum order.
=Complex Colours.=--We have demonstrated that single standard glasses
develop the three Dominant colours, Red, Yellow, and Blue, and that
pairs of equal standard glasses develop the three Subordinate colours,
Orange, Green, and Violet. In order to produce complex colours
two standard glasses of unequal value must be used. The degree of
inequality does not alter the spectroscopic names of complex colours,
variation in proportions being only a statement of degree.
Complex Name. High value. Lower.
To develop a Red-Orange insert a Red and Yellow.
" Yellow-Orange " Yellow and Red.
" Yellow-Green " Yellow and Blue.
" Blue-Green " Blue and Yellow.
" Blue-Violet " Blue and Red.
" Red-Violet " Red and Blue.
A reference to the circles, 7, 8, and 9, on the cards supplied with
the apparatus will show that all complex colours are members of the
same triad group, and experiments have shown that the six combinations
above are the only ones distinguishable in Nature, subject, however, to
unlimited variations in brightness or dullness. It remains to be shown
how these variations are effected.
Variations in brightness are produced by inserting with the two glasses
forming a complex colour the third spectrum colour, always bearing
in mind that it must be less in value than either of the other two.
The addition of the third colour has a dulling or saddening effect,
the degree of which is determined by the numerical value of the third
colour. The colour produced by the addition of the third colour may
be termed a saddened or dingy colour, the appearance being that of a
brighter colour seen in shadow.
Reviewing the foregoing, it is demonstrated that primary or dominant
colours are transmitted by a single coloured standard glass; the
secondary or subordinate colours are transmitted by two equal standard
glasses of different colours; the complex colours by two unequal
standard glasses of different colours. Saddened pure colours are
developed either by one coloured standard glass combined with two equal
standard glasses of different colours and lesser value or by two equal
standard glasses of different colours and a third of lesser value.
Saddened complex colours by three unequal standard glasses of different
colours. Greys, which are steps towards blackness, are produced by
three equal standard glasses of different colours.
It is well known that Colour Blindness is a defect in the vision often
involving the confusion of such utterly distinct colour sensations as
Red and Green, Orange and Violet, and many others as widely different.
In the cases of Red and Green the confusions are specially disastrous
should the subject be a railwayman or a sailor. It is not, however, so
well known that many cases of so-called Colour Blindness are in reality
cases of Colour Ignorance, and the capacity for distinguishing between
colours and shades is often latent, and only waiting to be developed by
Education.
When a child persistently misnames colours after having received an
amount of instruction sufficient to remove colour ignorance in a normal
case, the errors are probably due to some form of colour blindness.
Such cases should be registered for further examination, taking note of
the miscalled colours. It would be an additional precaution to change
the position of the colours in the windows of the apparatus, in order
to prevent the association of colours with their positions as first
given in the instructions.
_Note._--The paints which most nearly correspond to the colour
standards in the Colour Educator are tabulated below, the third column
containing their measured colour proportions when they are washed
thickly on white paper (_Whatman’s_).
--------------+---------------+-----------------------------------------------
| | Colour Composition in Standard Units.
Spectrum | Artists’ +-------+-------+-------+-------+-------+-------
Names | Names. | | | | | |
| | Red. |Orange.|Yellow.| Green.| Blue. |Violet.
--------------+---------------+-------+-------+-------+-------+-------+-------
| | | | | | |
Dominants: | | | | | | |
Red | Crimson Lake | 18·0 | 0·4 | ---- | ---- | ---- | ----
Yellow | Lemon Yellow | ---- | ---- | 6·6 | ---- | ---- | ----
Blue | Cobalt | ---- | ---- | ---- | ---- | 9·0 | ----
| | | | | | |
Subordinates:| | | | | | |
Orange | Chrome Orange | ---- | 7·2 | 1·6 | ---- | ---- | ----
Green | Emerald Green | ---- | ---- | 0·40| 5·4 | ---- | ----
Violet | Mauve | 3·3 | ---- | ---- | ---- | ---- | 8·2
--------------|---------------+-------+-------+-------+-------+-------+-------
In mixing two dominant colours (artists’ primaries) to develop
subordinates (artists’ secondaries), their relative colour depth must
be taken into account; for instance, Crimson Lake has a natural colour
depth nearly three times that of Lemon Yellow; therefore, in order to
develop a normal Orange nearly three times the quantity of Yellow must
be used, presuming that they were originally ground to an equal degree
of fineness.
It is desirable that a record of the children’s own painting should be
preserved, with a view to discriminating between errors arising from
Colour Blindness and Colour Ignorance, the former perpetuating itself,
and the latter naturally remedying itself. For this purpose painting
books containing sets of diagrams corresponding to the figures in the
foregoing pamphlet can be supplied.
Appendix II
THE POSSIBILITIES OF A STANDARD LIGHT AND COLOUR UNIT.[5]
[5] Reprinted from the _Journal of the Society of Dyers and
Colourists_, March, 1913. No. 3, vol. xxix.
The past attempts to standardize light and colour are mainly limited
to those radiant energies which excite light and colour sensations
under diffused daylight conditions, because in direct sunlight, and in
most artificial lights, there are other colour energies, which, unless
sufficiently modified by diffusion, disturb the colour readings. There
are also latent colour energies, which only become distinguishable
by special means. They do not, however, appear to influence diffused
daylight colour work.
The definition of a normal vision is one which agrees with a majority
of others. This definition has proved satisfactory up to the present,
as the normals are many and the colour blind few.
_Light Intensities._--There are two methods of determining light
intensities by means of a graded scale of light absorbents.
First. By total absorption of the light, when the intensity is directly
represented by the unit value of the absorbents required. This method
is applicable for low lights, internal surfaces, such as a desk, etc.,
where a standard light is not available for comparison.
Second. By the reduction of a standard light by absorption until it
equals the light of the object. In this case the standard must be
originally brighter than the object.
_Constants._--The first requirement in establishing a scale of light
and colour units is a means of co-relating visual sensations to a scale
of physical colour constants, in order to secure a power of record and
recovery. There is no natural scale available for quantitative colour
work, but artificial scales can be constructed, and made constants by
co-relation at different points with physical colour constants, and by
cross-checking the intervals between these.
The scales used in the “tintometer” consist of red, yellow and blue
glass, so graded in equivalents that combinations of equal units
transmit colourless light. Full details of these have already been
placed before the Society (see this _Jour._, 1887, p. 186, and 1908, p.
36).
=Scale of Luminous Intensities.= _The Light Unit._--The natural
terminals in a scale of luminous intensities are black and white, and
the first question which arises is what is black, and what is white?
as when used in a popular sense each term covers a wide range of
differences.
In the author’s sense the term black means total absence of light,
and the term white means a diffused daylight of given intensity, as
reflected from a lime sulphate surface. In this sense black and white
are the terminals of a scale of light intensities; the scale is divided
into units and fractions of units. The unit itself is physiological,
and is not in progressive accord with the mathematical light unit based
on the inverse squares of distance.
_The Black Unit._--Ideal black is practically obtained under daylight
conditions by viewing a hole in a box with blackened interior, so
arranged that no entering light can be reflected back to the vision.
The box used for this purpose is illustrated in Fig. 3, and has one
surface covered with standard white for the purpose of easy comparison
with the pigments. The standard black aperture (1) is in the middle.
The pigmentary blacks (2 to 10) are arranged over this, and the
pigmentary whites and greys (11 to 20) underneath, each being numbered
in accord with its intensity as tabulated.
[Illustration: Fig. 3.]
The degrees of blackness are the number of absorptive units required to
reduce the standard white to equal the pigments in each case.
Light Absorbed by Various Pigments.
---+---------------------+--------+----------+-------
No.| |Absorbed|Unabsorbed|Initial
| | Light. | Light. | Light.
---+---------------------+--------+----------+-------
1 |Black Hole in Box | 36 | -- | 36
2 |Optical Black | 20 | 16 | 36
3 |Lamp Black | 17 | 19 | 36
4 |Vegetable Black A | 17 | 19 | 36
5 | " " B | 14 | 22 | 36
6 | " " C | 15 | 21 | 36
7 |Indian Ink on Paper | 14 | 22 | 36
8 | " Solid | 12 | 24 | 36
9 |Boot Black | 11 | 25 | 36
10 |Black Lead | 9 | 27 | 36
---+---------------------+--------+----------+-------
This gives a working scale of colourless light intensities, the
terminals being black and white, with a range of 36 units.
_The Standard White._--White is the natural terminal of the luminous
end of the scale, and it is necessary to select a physical objective
white as a constant. Pure precipitated lime sulphate has been adopted,
and departures from the light intensity of this are recorded in units
of lessened light intensity throughout the scale, comprising all
degrees of colourless whites, greys, and blacks.
Strictly speaking, white is a qualitative term only, until the degree
of variation from the zero of the scale has been established. The
measured variation then takes its position in the scale of luminous
intensities according to its numerical unit value.
Light Absorbed by Various White and Grey Pigments.
---+--------------+--------+---------+-------
No.| Pigments. |Absorbed|Reflected|Initial
| | Light. | Light. | Light.
---+--------------|--------+---------+-------
11| Grey Paint E | 6·0 | 30·0 | 36
12| " D | 5·0 | 31·0 | 36
13| " C | 4·0 | 32·0 | 36
14| " B | 2·0 | 34·0 | 36
15| White Paint A| 0·7 | 35·3 | 36
16| White Paper D| 0·3 | 35·7 | 36
17| " C| 0·2 | 35·8 | 36
18| " B| 0·25 | 35·75 | 36
19| " A| 0·15 | 35·85 | 36
20| Chinese White| 0·006 | 35·994| 36
---+--------------+--------+---------+-------
As the scale is differentiated into hundredths of a unit, there can be
100 variations of white pigments in a single unit, each quite easily
distinguishable from the others.
Any definite mixture of black and white finds a position on the
diagonal of a chart whose co-ordinates are the black and white scales;
for example, the 20 measured pigments are charted on Figs. 4 and 5, the
latter being on an enlarged scale, as the whites would be too crowded
to be noted on Fig. 4.
[Illustration: Fig. 4.]
The merging of white into grey, and of grey into black is gradual,
having no strict lines of demarcation.
An example of this method of determining light intensities is
illustrated in Fig. 6 by the light intensities at which different
objects are discernible. The points of most interest are, that colour
is indistinguishable as such in lights below 15 units intensity; and
that ordinary work, such as reading a newspaper, requires for comfort a
minimum of 28 units.
_The Colour Unit._--The colour unit is physiological, and its
dimensions are determined by the dimensions of the colourless light
from which it is derived. This deduction is based on the experimental
fact that colourless light is a mixture of the six colour rays--red,
orange, yellow, green, blue, and violet--in equal proportion, as
illustrated in Fig. 7, showing that a white light of 20 units light
intensity is made up to the six colour rays, each of 20 units colour
intensity. This is demonstrated by the fact that any proportion of
any colour can be developed at will by means of the glass standard
scales already mentioned; it follows that the smallest disturbance of
equivalence between the composing rays results in the development of
colour.
[Illustration: Fig. 5.]
The above remarks apply to both simple and complex colours, and
the complex colours are always dichromes, being governed by
another physiological fact, which is: That the vision is unable to
simultaneously distinguish more than two colours in the same beam of
light. The order of their association is definite, and may be described
by saying that the combined two are always adjacent in their spectrum
order, red and violet being considered adjacent for this purpose. It
follows that all complex colours are binaries, and the only possible
combinations are as follows:--
Red with Orange.
Orange " Yellow.
Yellow " Green.
Green " Blue.
Blue " Violet.
Violet " Red.
[Illustration: Fig. 6.]
In the author’s colour nomenclature, a monochrome is qualitatively
described by a single term, and a complex colour by a combination of
two single terms. For a quantitative description, it is only necessary
to add the measured unit value to each term. When there is excess of
brightness, or a saddening factor, these also must be quantitatively
estimated.
The colours developed by means of these scales are governed by the same
law of selective absorption which governs the development of natural
colours, any of which can be matched and reproduced by means of their
established ray proportions.
[Illustration: Fig. 7.]
The governing law is simple, and may be stated by saying that the
colour developed is always complementary to the colour absorbed, not in
the generally accepted sense that their mixture necessarily makes white
light, but in the sense that they are opposite in the cycle of daylight
colours.
The dimensions of the unit are necessarily arbitrary, it was originally
selected as being a convenient depth for distinguishing differences,
the scale was then constructed by equal additions and sub-divisions;
the two essentials of a scientific scale being complied with, in
that the divisions were equal and the unit recoverable. The power
of recovery lies in the fact that different parts of the scale are
co-related to physical colour constants, which can be prepared in any
laboratory.
[Illustration: PLATE VIII ABSORPTION CURVES OF SIX DYES.
_To face page 76._] [_Lovibond, Colour Theories._
]
_Specific Colour._--The relationship of colour increase to intensity
increase in substances has hitherto been somewhat obscure. It has been
sometimes considered that they were in direct proportion, but in the
absence of a means of recording colour sensations, no definite results
were obtainable.
[Illustration: Fig. 8.]
Sufficient information is now available to warrant the formulation of
the following law: “That every substance has its own rate of colour
development for regularly increasing intensities, which, when once
established, becomes a constant for identifying similar substances
in future.” This is the meaning of specific colour, and when a
series of measurements at regularly increasing densities of a given
substance have been made, the specific colour rate of that substance
is established. This can be charted in curves and used as a basis for
estimating quantities, properties, changes of condition, differences in
value, detecting adulteration, etc.
_Applications._--The author has permission to use the names of several
gentlemen who have used the tintometrical scales for various purposes.
Sir Arthur H. Church, F.R.S., has employed the tintometrical standards
for the purpose of registering the colours of certain wild flowers.
Sir Boverton Redwood has used the scales and system for petroleum
investigations. At his instance the specific colour rate of petroleum
was established, and the several composing colours plotted in
curves, as in Fig. 8, where the ordinates represent the scale of
units irrespective of colour, and the abscissæ the scale of strata
thicknesses.
The measurements were made at two-inch intervals, and the four
perpendicular lines are at the colour points selected for valuing
the four distinguishing marks, technically known as “Water White,”
“Superfine,” “Prime,” and “Standard.” Intermediate qualities find their
position in the scale of curves according to their measured colour
values.
This method of standardizing commercial values has also been adopted by
the International Tanners’ Association, the Inter-States Cotton Seed
Oil Association, and other oil industries. Also for scale, solid fats,
and such substances as can be easily melted and measured by transmitted
light.
=Varying Effects of Different Lights.= _Pathological
Applications._--The law of specific colour development was made use of
by Dr. George Oliver in determining the degrees of hæmoglobin in the
blood. The method is fully explained in his Croonian Lecture before the
Royal College of Physicians of London, July 11, 1896.
_Detection of Forgeries._--The system and apparatus is used by
Professor A. S. Osborne, Examiner of Questioned Documents, New York
City, for determining the variety of ink, the age of the writing,
and the detection of forgeries. A full description of the process will
be found in his work entitled _Questioned Documents_, published by the
Lawyers’ Co-operative Society, Rochester, N.Y.
[Illustration: PLATE IX SIX ANILINE DYES.
CURVES ILLUSTRATING THE RATE OF FADING BY EXPOSURE TO LIGHT.
_To face page 78._] [_Lovibond, Colour Theories._
]
The application to chemical analysis is too well known to require
enlargement here.
[Illustration: Fig. 9.]
_Dyes._--As an example of the use of the system in the valuation of
dyes, Fig. 9 illustrates the specific colour curves of four samples
of Methylene Blue. No. 1 was priced at 5_s._ 9_d._ and No. 2 at 5_s._
per lb., Nos. 3 and 4 were not priced, the solutions were measured in
percentages from 0·001 to 0·048 in distilled water. To find the cost
per unit of colour in the priced samples is only a question of simple
arithmetic, which furnishes data for the valuation of the unpriced
samples.
The yield in the dye vat may not be in direct relation to the solutions
in water, the establishment of this is a question for the expert, and
presents no apparent difficulty.
The use of the scheme in recording the degree of fading of dyes has
been previously dealt with in the _Journal_ (_q.v._, 1908, p. 36).
_Limitations and Precautions._--It has been shown that we have
analytical control, within certain limits, of light and colour under
daylight conditions.
The general limits for colourless light range from total darkness to 28
units, when the unabsorbable red ray comes into evidence.
For colour, the general limits range from 28 to 18 units, between 18
and 15 all colours become indistinct, but at varying rates, below 15,
colour is not distinguishable.
The principal disturbing conditions in making observations are _want
of colour education_ and _insufficient diffusion_. In the case of the
latter, the first evidence is the disturbance of constancy by the
penetrating red ray. A partial remedy is to interpose a white diffusion
screen, such as tissue paper.
_Time of Observation._--This should not exceed five consecutive
seconds, as the keenness of perception decreases by time, but varies
for different colours.
_Angle of Incidence._--Sixty degrees is safe for most solids, but for
bright or polished surfaces, such as varnishes, polished metals, etc.,
the angle must be lessened as the degree of smoothness increases. For
very rough surfaces, such as loosely woven stuffs, etc., care must be
taken that the lay of the fibre is uniform.
_Distance from the Object._--Ten inches has been adopted for general
work, but certain visions require more or less as their focus varies
from normal.
[Illustration: PLATE X COMPARISON CURVES OF HEALTHY AND DISEASED BLOOD.
_To face page 80._] [_Lovibond, Colour Theories._
]
_Unabsorbable Colours._--In addition to the daylight colours already
dealt with, there are, in direct lights, colours which do not obey the
laws of absorption governing those of diffused daylight.
The work already done on these unabsorbable rays has only been
incidental, where they happened to interfere with the standardization
of diffused daylight colours. The sensations excited are red and
violet. They blend, producing red-violet mixtures, but in unequal
proportions, the red being dominant.
The red is developed in intense lights by constant interception of
neutral tint absorbents. In the case of a 4-volt incandescent light,
the first absorption simply reduces the light intensity without
developing colour; the light is colourless up to 14 units. At 16
units the light begins to assume a reddish hue, which rapidly becomes
a brilliant intense red by further interceptions of neutral tint
absorbents.
Violet is developed by constant absorption by blue standards, which
grows in intensity by successive additions up to about 120 units.
Beyond this point the brilliancy decreases.
Preliminary experiments point to this ray as fatal to vegetation, and
presumably also to lower forms of organic life.
There remains a factor of considerable importance which has not yet
received the attention it deserves, the physiological changes resulting
from environment.
This aspect of the question has come under the notice of the author
by measuring the vision of experts who excelled in given hues. It was
generally found that their vision was sensitive to a small increment of
their particular colour in harmonies where it was silent to a normal
vision.
In seeking an explanation of this phenomenon there are at least three
possible lines for working:
First. Is the vision naturally more sensitive to that particular energy?
Second. Is it by careful education?
Third. Is it an unconscious adaptation to surroundings, such as other
organs undergo under changes of environment?
[Illustration: PLATE XI SPECIFIC COLOUR CURVES OF HEALTHY AND DISEASED
HUMAN BLOOD
_To face page 82._] [_Lovibond, Colour Theories._
]
Appendix III
THE APPLICATION OF THE NATURAL LAW OF SPECIFIC COLOUR RATE BY
DR. DUDLEY CORBETT TO THE EXACT MEASUREMENT OF X-RAY DOSAGE.
Dr. Dudley Corbett.
The gradations in the tint given by the Sabouraud-Noiré pastille
when exposed to X-rays are so fine, especially in that region of the
colour scale where lies the erythema dose, that many have felt the
want of a more accurate means of reading these tints, as well as a
series of reliable standards for comparison. Hitherto the only methods
at all generally used have been Hampson’s radiometer and Bordier’s
radio-chronometer, the former in this country, and the latter on the
Continent. Hampson’s instrument has two disadvantages: it can only be
used with electric light, and the standards are made of tinted paper
liable to get soiled, and to vary slightly with the changes in the
pigment employed. Its advantage is that it may be used as a sliding
scale, thus economising the pastilles. Some persons, however, have
considerable difficulty in reading the tints, the scale rising only by
gradations of 1/4 B.
In the construction of any such instrument, the really important
point is to obtain a reliable standard for Tint B--_i.e._, the normal
epilation dose. The tints on the Sabouraud card itself are not always
identical, some representing a dose which will only just epilate,
others an almost dangerous dose for unfiltered rays. The Tint B, which
is the standard, allows a margin of error of 20 per cent. on either
side. In other words, 4/5 B will almost always epilate, while 1-1/5
B is nearly the limit of safety. This observation is in accordance
with the experience of other workers on this subject. In my instrument
Tint B has been obtained by measuring the pastille with Lovibond’s
tintometer immediately after exposure to the X-rays. The pastille was
turned to a tint corresponding to an epilation dose which was known to
be safe, as proved by clinical results. This tint was measured directly
both by daylight and by artificial light from an 8-candle power carbon
filament lamp with frosted glass shade. I am indebted to Mr. Dean for
suggesting the use of Lovibond’s instrument for this purpose.
The methods employed in the experimental work have been described in
the _British Journal of Dermatology_ for August, 1913. By using a
very constant focus tube and averaging a large number of readings and
correlating the results with those obtained in clinical practice, we
were able to construct the curves indicating the colour developed by
the pastille. In these curves, shown in Fig. 10, the ordinates are the
Lovibond colour units, the abscissa the time during which the current
was actually passing. When using an interrupter working at a constant
speed, the actual time was taken, otherwise the number of current
interruptions as measured by a dipper tachymeter was used.
As was to be expected, the daylight and electric light curves were
quite different. In each case the standard yellow glasses employed were
kept constant throughout the curve, that for daylight being 15 units,
that for electric light 13 units. When these were combined with blue
and red glasses in varying units and fractions of a unit, they gave
a colour range which matched the pastille exactly in the changes it
undergoes from the unexposed condition to the 2 B Tint.
The curves are plotted in accordance with Mr. Lovibond’s practice--that
is, not as a direct representation of the standard glasses used, but as
showing the colour sensation received by the eye.
[Illustration: Fig. 10.]
First, as to the daylight curve: In order to match the unexposed
pastille we interpose between the pastille and the observer’s eye
a yellow and a blue glass, plus a certain amount of neutral tint
(composed of three equal colour units). Thus the colour sensation
received is a yellow green, together with a certain amount of white
light. As the pastille darkens under irradiation, both the green and
the white light disappear, until at a point just below the 1/2 B dose,
there is no other colour present but yellow. After this red glasses
are required--_i.e._, the colour sensation is a yellow orange, which
gradually deepens owing to an increase in the proportion of red. The
yellow curve thus rises till just below the 1/2 B point, and then falls
as the orange increases.
Next, as to the electric light curve. The unexposed pastille has but
a trace of green, which is soon lost. The orange begins much earlier
than in daylight, and thus at Tint B has reached a higher point than in
daylight. From this point the orange and yellow parts of both curves
run practically parallel with one another up to 2 B. Beyond 2 B the
readings become more difficult. I have not determined the point when
no more colour develops, as it has no great practical value, though it
might well be of interest from a physico-chemical standpoint.
In my radiometer the standards are composed of the Lovibond standard
glasses in combination. The apparatus itself consists of an optical
instrument or viewing box. This is divided by a central partition,
so that on looking through the eyepiece one sees a white background
through two small circular apertures. On one side, level with the
background, is a fitting to take the pastille in its holder. On the
other side is a groove in the instrument itself for the insertion of
the standard glasses. A similar groove is fitted on the pastille side
of the instrument to take neutral tints if required. The colour of the
pastille as seen by reflected light can thus be compared with that
obtained by transmitted light through the standard glasses seen against
the white background. A difference of 1/5 B or 1 H is quite easily
perceivable.
It is usually of no great importance to obtain extremely accurate
measurement of the smaller fractions below 1/3 B. Where this is
necessary, neutral tints must be used when working with daylight. With
electric light these are unnecessary. When required the neutral tints
are interposed between the pastille and the eye to absorb the white
light reflected from the pastille. The neutral glasses required are 1·5
for the unexposed pastille, 0·6 for 1/4 B, and 0·2 for 1/3 B. These
values are subject to slight variations due to changes in the varnish
of the pastille emulsion. The difficulty can always be avoided by using
electric light, where a trace of neutral tint is needed only when
matching the unexposed pastille--an unimportant point.
_Method of Use._--The choice of daylight or artificial light is a
personal matter, but one should practise reading the scale with both.
The use of the instrument shows that the pastille fades very nearly as
quickly under electric light as it does under daylight. The following
precautions should be observed: In daylight work in a good white
light, avoid shadows and yellow light of any kind. With electric light
use an 8-candle power carbon-filament lamp with frosted glass and a
suitable white shade so arranged that the pastille is 8 inches from the
lamp. No other light should be allowed to reach the pastille during
examination. A low power metal-filament lamp may be used, but greater
accuracy will be obtained with a carbon-filament lamp which was used
for the experimental work. The lamp should be discarded as soon as
the light becomes yellow from prolonged use. Whether in daylight or
electric light, the examination must be rapid to avoid the fading of
the pastille. When it is desired to give an accurate 1 B dose, it is
better to put up the 4/5 B standard first. It is then easy to calculate
how much more exposure is required for the extra 1/5 B. It is important
to adjust the pastille carefully so that none of the unirradiated green
portion is visible through the small aperture, as this will upset the
reading. In very accurate dosage new pastilles should be used, as a
bleached pastille never returns exactly to its original tint. When such
a bleached pastille is irradiated the colour changes start a little
farther down the curve, and thus the tint for a given dose must be
taken a little above the normal tint. This increase is very slight,
but is nevertheless quite appreciable, and may amount to as much as
5 and 10 per cent. Even then the margin for error is ample in the
neighbourhood of the B tint, and if a pastille is not used more than
three times, and is well bleached in daylight after each exposure, no
serious error is likely to occur. A standard white background should
always be used, and discarded for a new one when it gets dirty. The
colour standards usually provided are those in common use--namely,
1/4, 1/3, 1/2, 4/5, 1, 1', and 2 B, but it is quite easy to make up
standards for any point on the curve. The symbol “B,” as the erythema
or epilation dose, has been retained, as it was thought inadvisable to
add to the number of such symbols already existing.
To sum up:
1. The experimental work has determined the exact colour changes
occurring in the Sabouraud pastille when exposed to X-rays.
2. These experiments have established a permanent standard for Tint
B, which matches the pastille exactly, does not fade, is easily kept
clean. These coloured glasses can be readily and accurately reproduced,
as they are standardized spectroscopically by a firm who specialise in
such work. The standard will therefore remain constant so long as the
Sabouraud emulsion remains unaltered.
3. Glasses may be prepared of the correct tint for any fraction or
multiple of this dose up to 2 B or 10 H.
4. Either daylight or electric light can be used.
5. The optical instrument itself, by cutting off extraneous light,
greatly assists the colour comparisons, so that the practical error
need never exceed 10 per cent.
INDEX
Abnormal light, 13
Analysis of white light, 11, 13
Arbitrary scales, 9
Artists and scientists, 1, 2
Beam of white light, 7, 11
Black units, 16, 23
Black, ideal, 15, 11
Blood curves, 33
Code of laws, 7
Colour charts, 31, 39
education, 59
equivalence, 10
nomenclature, 17, 21
qualitative, 17, 19, 24
quantitative, 24
scales, 20, 29
standardization, 20, 69
theories, 2, 3
Coloured surfaces, 44
Colours, complex, 18
sadder than standards, 25
simple, 17
Corbett, Dr. Dudley’s, radiometer, 83
Daylight colours, 24, 84
measurement, 14
Diffused light, 10, 43
Direct lights, 46
Dulled colour, 10, 18-26, 36
Equivalent colour units, 7, 9, 29
Fog, whiteness of, 10
Glass standards, 20
Human blood curves, 82
Laws of colour, 7
Light, 11, 13, 42, 45-46, 81-84
abnormal, 8
brighter than standards, 14, 27
direct, 46
for colour work, 42
intensities, 42, 43
white, 14
Lovibond’s new colour theory, 5
Matching colours brighter than standards, 18
complex colours, 17
Measuring and naming colours, 21
Monochromes, 17, 19
Munro, Dr., 34
Neutral tint, 18, 24, 26
North light, 15
Past theories, 3
Photographic energies, 53
Physical colour constants, 8
Pigmentary black, 15, 71
Primary colours, 1
Prismatic spectrum colours, 29
Qualitative analysis of colour, 17, 19, 24
Quantitative analysis of colour, 24
Radiometer, 83
Rate of colour absorption, 8
Rays, six colour, 7
Red ray, 41
Scales, arbitrary colour, 9
coloured, 10, 20
cross checking of, 9
Scientists and artists, 1, 2
Sea fog, 10
Specific colour, 32
Spectrum colours, 4, 18, 36
Three colours, 10, 57
Time, appreciation of colour by, 42
Ultra violet, 32, 40
Unit, checking of, 9
Unit, neutral tint, 18, 24, 26
Wave length position, 37, 38
White light, 11, 13
Butler & Tanner Frome and London
Transcriber’s notes:
In the text version, italics are represented by _underscores_, and bold
and black letter text by =equals= symbols. Superscripts are represented
by ^{} and subscripts by _{}
Missing or incorrect punctuation has been repaired. Inconsistent
spelling and hyphenation have been left,
In the html version, dittos have been replaced by the repeated text so
that text aligns for easier reading.
Very wide tables (V and VI and VII)have been split to fit on portrait
oriented pages.
The following mistakes have been noted:
p. 21. NiSO_{4}7H_{2}O, tem." is formatted inconsistently. Left as
printed.
p. 40. Fraunhoper is almost certainly a typo for Fraunhofer.
Left as printed.
p. 46. interpretated changed to interpreted.
p. 55. In Paper 6 of table, Red screen, yellow entry, from alignment and
logic, the 6 should be .6. Left as printed
p. 61 conditonschanged to conditions.
End of Project Gutenberg's Light and Colour Theories, by Joseph W. Lovibond