Popular Science Monthly 



103 



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 



