The Salt Lake herald. [volume] (Salt Lake City [Utah]) 1870-1909, 1900-04-24

ward side Of a ship is comparatively
smooth, if not quite so; and sound
shadows occur behind large obstacles.
Water waves bend around a stake
just as sound waves bend around
small obstacles. Light waves do the
same thing. but, as the light waves are
so very short, the obstacle must be
very small to produce the effect of the
stake in the water A body of the di-
mansions Of hair or a fine line on
glass viii do this, and when the diff
ferent sets Of waves meet on the other
side, they interfere, causing added
light where crest meets crest and de-
stroylng each other where crest meets
trough. s'hus alternate bands of light
and darkness are produced which may
be clearly seen under favorable cOnd5-
tions with the proper apparatus. This
effect is very greatly increased by rul-
ing great many lines close together,
the lines all being of the same width
and separated by spaces of the same
width. Such a set of ruled lines, on
glass or on other transparent or high
~ rejecting body, is called "diffrac-
ton grating
Action of the Diffraction Grating.
If the image of a light Of one color
only. such as the yellow sodium fame.
is formed by a lens upon d screen,
and a grating put in front OF the lens,
there appear other yellow images on
either side of the central image sep
arated from it and from each other by
dark spaces. If the color Of the light
is blue the spacing Of the side images
is smaller, each blue image falling
nearer the central line than the CORE
spending yellow. and images from a
red source would have wider spacing
than the yellow. In this way white
light is broken up by a grating and
spread out into a series of spectra on
either side of the central line. More
over, the spacing lines of the grating
determines the distances separating
the spectra, finer ruling causing the
images to spread out more widely. Ev-
identity, then, by using gratings of diff
ferent fineness of ruling we may cause
the spectrum produced by the second
the spectrum nearest the central image
since it is the brightest-tO lap upon
| the pectrum produced by the first, and
that of the third grating to lap upon
the two others. Indeed. the ruling Of
| the gratingS may be carefully deter
mined, so that the red from the first,
| the green from the second, and the
violet from the third will all fall upon
the same spot. An eye placed at that
| spot would see red, green, violet or
any Of their compounds, including
white, according TO the relative
| strengths of the simple component col
| ors. 1t is this action which is the foun-
| dation of Woods method.
The Process.
! From what has been said it is evil
| dent that this method is in a sense a
| three color method, but the primary
! cOlOrs used are pure spectrum cOlors
/ which by an ingenious arrangement
are produced automatically in the film
| itself. In the finished positive the red
| component at each point is represent
] ed by portion of the grating pro-
/ ducing red at the view point; the green
component by a portion of the green
| producing grating and the violet COM
pOnent by portion Of the grating pro
| ducing violet.
Three negatives are first obtained
under color screens, as in the ordinary
| three-colOr process, and a positive Is
| made from each. Then the red posi-
! tive, which of course shows no color
with the red grating in contact. is
! used to make an exposure on a bi-
| chromated gelatin plate. An impress
sion Of the grating lines is made on
those parts of the plate lying under
| the clear parts of the picture. A sim
liar exposure on the same plate is Their
made under the green positive, and the
green grating. and a third exposure
with the violet plate and the violet
| grating. The plate is then washed II
warm water. dried, and the picture i:
done. Copies may then be made from
this plate at a single exposure by sun
light printing in the usual way, anc
all copies on bichromated plates will be
positives-
A viewing apparatus is necessary tC
see the picture in the right colors. i
consists only Of a lens and a per
forated screen for bringing the eyes
into the right places.