Page:Popular Science Monthly Volume 89.djvu/117

 Popular Science Monthly

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��directly back of the lens is known as the "prccn" plate; while the other at right angles is referred to as "red." This arises from the fact that light rays reach- ing the "green" plate must first jiass through a green filter, while those falling on the "red" plate are correspondingly filtered by a red glass. The "green" plate is intended to record at the green portion of the spectrum, while the "retl" is sensitive to those at the opposite end.

The manner in which the image is convened to both plates is interesting. Thus, Mr. Brewster mounts a few inches back of the camera shutter, a mirror called the "Swiss Cheese" plate, its sur- face being at a 45° angle with the plane of the lens. The mirror is thus strangely named because it is full of holes, which serve to permit parts of the image to pass through to the "green" plate; the remain- der being rellected by the solid part to the "red" plate. Inasmuch as images fil- tering through the holes overlap after passing the mirror, a complete picture is thrown on the "green" plate — and not a spotted one, as might be e.vpected because of the holes. Likewise the solid portion thnnvs a com- p 1 e t e i m a g e on the "red" plate. Di\id- ing the light between the two plates in this manner of course lengt li- ens somewhat the time of ex- posure neces- sary ; other- wise no other effects are or- dinarily no- ticeable.

The same effect can be obtained in many other wa>s. Thus, i n w hat is known as the "kodachrome" process a plate is employed which, instead of being perforated with Swiss cheese holes, is thinly platinized, so that it can both reflect and transmit light.

���The "Swiss cheese" mirror. The dotted lines indicate the size of the holes on the reverse side

��It is understood of course that nega- tives obtained with the Brewster, "Koda- chrome," and similar instruments are of the ortlinary black-and-white variety — not colored in any way. The "green" plate differs from the ordinary negative only in the fact that it is especially dense where colors at the green end of the spectrum predominated, while the "red" plate likewise records densely roseate hues. From these two negatives positives are made on other plates by ordinary processes of contact printing. The image on the positi\'c from the "green" plate is dyed red and that from the "red" plate green. The two positives are then placed face to face, and the image on one registered with the image on the other.

Hold the combined plates up to the light, and you can see the photographed object in its natural colors. It stands out from the background as striking as the original. The effect is startling, indeed.

Why Two Colors Must Be Employed

The rea.son for coloring the "green" positive red, and the "red" positive green, as mentioned in the foregoing, is

rather elusive and at the same time par- ticularly inter- esting. Con- sider for in- stance the case of a red rose on a back- ground of rreen leaves. The "green" negative upon development will be almost l)lack where the green leaves appear on the plate, while the rose will be almost transparent. Similarly with the "red" negative, the rose will appear dense, while the green is recorded as a transparent area.

Positi\-es from these two plates will in each case of course be just the opposite

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