The Optical Magic Lantern Journal (March 1901)

The Optical Magic Lantern Journal and Photographic Enlarger. 33 The spots may also be made with the white ‘‘ink,” using the round end of a pencil (charged with paint) rubber-stamp fashion on the masks. Both these spots and the stamp edge kind being under the cover glass, cannot meet with accidents and get rubbed or cleaned off. White-faced masks should be selected carefully, as they are not always light proof with the stronger lights. Cover glasses do not seem to receive much care in selection, judging from those slides passing through my hands lately. In many cases the picture is marred by bubbles and flaws in the cover and more rarely in the glass of the slide itself. It is, therefore, quite as well to look to these points if the slides are meant for competition or any very particular purpose. Another little point which frequently escapes attention is to get the masks with straight edges quite parallel with the roofs and walls of any buildings in the scene. If one trimmed a print out of straight it would be noticed at once, but many slides I get are out in this respect, and it is a fault very noticeable on the screen. Other mask faults, not perhaps so frequently met with, but still too often seen, are jagged edges, pin-holes, circles flattened or ovals not true. Masks should be scanned and faulty ones destroyed at once. , All these points may only seem grumbles, but if the lantern is to keep its place and advance in this critical age, the smallest details must be attended to. sis COI Ke S—< The Optics of Trichromatic Photography.—Part IV. Tar Traitt Tavtor Memoria LECTURE, Continued from page 23. Cros also proposed both positive and negative synthesis: He at first appeared to accept red, yellow, and blue as the primary colours of light, and described methods of positive synthesis by triple lantern projection, by application of the principle of the zoetrope, by an arrangement of transparent reflectors, and by a prismatic device, employing in each case photographic positives from the original negatives, and red, yellow, and blue lights. For the production of colour prints he said the same negatives could be used, and the prints made in the ‘“‘antichromatic ’’ colours, ‘‘ green, violet, and orange.” Cros here clearly avoided the mistake of Collen and Du Hauron of trying to record two primaries in each printing negative, and it is remarkable that his printing colours, “green” and ‘‘ violet’ (purple), are just as near to the true printing colours, minus red, and minus green, as are true blue, and red. Afterwards, in the same article, he expressed the opinion that it might be better to make the negative by ‘‘green, orange, and violet” rays, and the prints in their ‘‘ antichromatic * colours, “red, blue, and yellow.’ Cros did not recognise red, green, aud blue as the correct triad of primaries, nor that the “blue,” and ‘‘red,’”’ or ‘green,’ and ‘violet,’ printing colours should be green-blue and crimson-red. There was no suggestion of anything so definite as analysis by colour-curve screens to be followed by positive synthesis with pure colours. Cros's suggestions were generally of a somewhat speculative character, and he amusingly disclaimed any wish to submit himself to the ‘painful '’ labour which he could foresee would be necessary to arrive at a practical realisation of colour photography by such a system. He said he preferred to show the way, and to claim the credit after somebody else had done all the bard work. By this decision he proved himself to be, from a material point of view, one of the wisest men who ever attacked this problem, and this being his position, he could afford to be somewhat vague—the more so the better; but some of his suggestions now appear wonderfully acute and prophetic, and are worthy of special notice. For instance, Clerk-Maxwell and Collen both recognised the necessity for colour-sensitive photographic plates in order to practically realise their ideas, and Du Hauron was content to give enormously prolonged exposures so as to utilise the extremely feeble colour sensitiveness of ordinary photographic plates. Cros, who also recognised this diffiulty, pointed out that a photographic plate can only be acted upon by light which it absorbs, and said he thought it might be possible to make the plates colour sensitive by incorporating suitable dyestufis. In this publication he clearly anticipated the ‘principle ’’ of ‘* optical sensitisers,” as it was afterwards stated by Dr. Vogel, and although he appears to have thought that the mere colouration of the film might serve to make the plates colour-sensitive, it is a .remarkable facs that he said a search should be made among a class of dye-stuffs, some of which actually do confer colour sensitiveness to bromide of silver plates ! Cros not only anticipated Du Hauron in the matter of actual publication, but was quite as fertile in original suggestions, and was, I think, more of a scientist, if less of a mechanic, than his rival. I believe the names of Clerk-Maxwell, Harry Collen, Baron Ransonnet, Louis Ducos Du Hauron, and Charles Cros complete the list of independent inventors of trichromatic photography. Although Clerk-Maxwell was the first, there is in the original publications of Collen, Du Hauron, and Cres, internal evidence of independent conception of the idea. Assuming that I have correctly set forth the essential conditions of success in trichromatic photography, and that I have fairly represented the publications of the original inventors, propositions by which I am prepared to stand or fall (I have recently studied the publications of Du Hauron and Cros most carefully), it follows that, brilliant as were the conceptions of these inventors, they failed to recognise requirements essential to success; and I do not hesitate to say that this is the true explanation of the discredit into which this idea had fallen after many attempts to reduce the methods to practice. Du Hauron and Cros continued to try to perfect and exploit their ideas, but made very little real progress that I have been able to discover, except that Du Hauron made distinct improvements in cameras for making the colour records, and Cros, in 1879 (The Review of Games, Arts, and Sports, February 15th, p. 221) had definitely settled upon “orange, green, and violet’ as the primary colours of light, and stated that the prints should be