IMPROVEMENTS IN OPTICAL INSTRUMENTS FOR VIEWING COLOURED SIGNAL LIGHTS
Year: 1877

page_text: PROVISIONAL SPECIFICATION left by Thomas Gray and John Macfarlane
Gray at the Office of the Commissioners of Patents on the 14th February
1877.
THOMAS GRAY, of Rokesley House, St. Michael's Road, Stockwell, in the County
of Surrey, and JOHN MACFARLANE GRAY, of No. 127, Queen's Road, Peckham, in
the same County. "IMPROVEMENTS IN OPTICAL INSTRUMENTS FOR VIEWING COLOURED
SIGNAL LIGHTS."
The colours employed for signal lights are red and green, or colours which are
approximately red and green when seen at night. Red and green are complementary
colours; a certain shade of green with a certain shade of red by their mixture
produce white. Coloured transparent media transmit certain coloured rays more
easily than others. Red glass transmits red easily, and nearly absorbs all the other
rays. Green glass transmits green rays, and nearly absorbs all the other
rays; a green light viewed through a green glass is therefore but little diminished
in brilliancy, but when viewed through a red glass it is scarcely visible or is
invisible. In the same way a red light easily seen through red glass is scarcely
visible, or is invisible when looked at through a green glass. All this is well known
as the principle of absorption of light by coloured transparent media.
According to our Invention we embody this principle in the construction of
optical instruments for viewing and distinguishing signal lights which are approxi-
mately complementary colours, say such lights, for example, as are carried as side
lights by vessels at sea. For this purpose in one form of our Invention we combine
in one instrument a red glass and a green glass, so arranged that when the instru-
ment is properly used it must present the red glass always to one eye and the green
glass always to the other eye of the user, say, the red glass always to the left eye,
and the green glass always to the right eye. A light that is clearly seen by the
right eye, and is scarcely seen or not seen at all by the left eye, will then be known
to be a green light, and conversely a light seen only by the left eye will be known
to be a red light. This arrangement of red at the left eye-and-green at the right
eye is specially advantageous for navigators; the light that is seen clearly by the
right eye is the light on the starboard or right side of a vessel, and the light clearly
seen by the left eye is the light on the port or left side of a vessel.
One form of our Invention is a pair of spectacles in which we combine a red glass

page_text: for the left eye, a green glass for the right eye, and an arch of wire on the upper
edge to prevent the instrument being put on the wrong way.
When our Invention is applied to binocular glasses, according to one form, we so
construct the parts containing the coloured glasses that when the binocular is held
one way the coloured glasses are not visible, and the binocular is then in its action
just an ordinary clear night glass. The coloured glasses are in small binged frames
near to the diaphragms of the binocular, and hang vertical when it is used to dis-
criminate coloured lights, and they fall horizontal and out of view when the binocular
is required merely as a clear night glass. Otherwise the coloured glasses may be in
slides, falling into view or out of view, according to the way in which the instrument
is held.
:
In another form of construction för binoculars or for telescopes we apply a red
and a green glass to each tube, and these are fixed on the same axis, so that either
the red or the green can be turned into view as desired, and when not turned into
view the coloured glasses are entirely out of view.
We also combine in one instrument two pairs of spectacles, one with both eyes
red glass, and the other with both eyes green glass, so marked by form or construc-
tion that the observer, even if colour blind, will always know which of the colours
he is using.
The objects of our Invention are,-
First. To enable a man to find out whether he himself is colour blind.
And, Secondly. To enable the distinction of signals by colour to be deciphered
in case of doubt by visibility or invisibility absolutely.
Our Invention is therefore meant to assist all eyes, and to give to those who are
colour blind the power of distinguishing between coloured signals. A white light
is seen with nearly equal brilliancy through red and through green.
When binoculars are used with one colour at each eye a white light will be seen
generally as two lights, one red and one green, partly overlapping, but some eyes
have the power of combining the two impressions into one nearly white light. It
is evident therefore that white lights will by the use of our Invention be also readily
distinguishable.