132 



DISCOVERY 



for paper. There an enterprise was created which 

 should go on indefinitely year by year. 



In Germany every slope that is too steep to plough, 

 every acre too cold to farm, is planted with suitable 

 trees. When cut, these are replaced by a definite 

 rotation of varieties, and thus an immense and profitable 

 industry brings benefit to the State. Every year, timber 

 augments in price as the civilised population grows, 

 and its appetite for wood is continually larger and 

 larger. We are not far from a timber famine, so it 

 behoves every Government to follow the good example 

 set us by our late enemies, who now find, with a 

 collapsed exchange, the enormous advantage of having 

 a good timber supply at their own doors. 



The Improved 

 Chromoscope 



By Daryl Klein 



The chromoscope is not described in the dictionary. 

 It has escaped the attention even of many who are 

 tolerabh" weU acquainted with the numerous -scopes 

 one finds depicted in scientific textbooks. In optical 

 research circles, the functions of this instrument are 

 not uncommonly known. And it is true that its name 

 gives a clue to its applicability. Nevertheless it has 

 remained discreetly in shadow for some sixty years. 



In 1S62, when the history of the chromoscope 

 begins, we can imagine an observant optician gazing 

 into a shop window. As he stands admiring its con- 

 tents, the sun comes out, and great is his astonish- 

 ment to see the traffic in the street moving, apparently, 

 in the shop window. Fact or phantasy, wonderful 

 enough in all truth to set him thinking, but a moment 

 later behold him bewildered to observe a top-hat 

 poised on the moment-since-bare head of a wax 

 mannequin. 



It ma}- be admitted that our optician was favoured 

 in the form of his illusion, but nearly everyone can 

 tell of the same experience. It arises, as the optician 

 no doubt reasoned, from the fact that the window 

 reflects as well as transmits light. The two images 

 so produced appear coincident in space, although the 

 rays which make the two objects visible reach the eye 

 by quite different paths. 



This is the underlying principle of the chromoscope, 

 a principle employed in " Pepper's Ghost " and 

 numerous other optical illusions which delighted 

 Victorians. The shop window serves as a two-picture 

 chromoscope, although perhaps not a very good one. 

 It is clear that, if our optician had stood on the far 



side of the street, the hat on the mannequin's head 

 would have looked ridiculously small. In order to 

 make both objects appear on the same scale, the optical 

 paths by which they are viewed must be equal. And, 

 we can conjecture, it was this reasoning which led 

 Ducos du Hauron in 1869 to construct the first chromo- 

 scope intended for use in connection with three- 

 colour photography. In Paris in the same year, 

 Charles Cros described a " photo-chromoscope " which, 

 according to him, would prove of considerable service 

 in finding a " solution du probleme de la photographic 

 des couleurs." 



For a reason which will be touched upon later, no 

 further application for the chromoscope was found. 

 F. E. Ives, whose name is associated with the 

 "original " chromoscope (1895), was himself an advo- 

 cate of its use in colour photography, and his simple 

 design has served as a model for most chromoscopes 

 constructed since. 



Fig. I shows the essential part of Ives's instrument. 



Fig. I.— .«v early chromoscope. 



A, B, and c are sheets of red, blue, and green glass 

 respectively, on which the three positives are placed. 

 H, D, and E are transparent mirrors, and the three 

 positives are so arranged that the distance traversed 

 by the light in passing from each positive to the eye- 

 piece is the same. So that the three dift'erently 

 coloured images appear to coincide and give rise to a 

 single-coloured image. 



It is conceivable that, using this method, we could 

 build up an instrument to take almost any number 

 of slides, but, if transparent glass mirrors are em- 

 ployed, a serious difficulty arises. Reflection takes 

 place at both surfaces of the glass, and each image 

 is therefore accompanied by a secondary image, 

 slightly smaller and farther from the eye, and con- 

 siderably fainter than the primary image. In the case 

 of the three-picture chromoscopes, these images are 

 not of very great importance, but the multiplicity of 

 images arising from the cause in, say, a ten-picture 

 chromoscope would be very distracting. In addition. 



