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Interference, examples of which may be found in peacock's 

 feathers, some stones, butterfly wings, beetles, and the ever 

 popular soap bubble, in none of which is the colour due to the 

 presence of a pigment. We may select the soap bubble as 

 perhaps the best illustration. Although the film may appear 

 to be of one substance throughout it is in fact irregular 

 in thickness, and it is through this irregularity and the 

 extreme thinness of the film that we see the colours and even 

 pictures of surrounding bright objects. The principle has been 

 applied to the production of natural colour photography, and it 

 is the only principle to which this term can apply. A diffraction 

 process of colour photography was expounded by Professor Wood 

 in 1899, but nothing practical came of it, so those workers 

 who had directed their attention towards the pigmenting method 

 were left masters of the field. 



In taking a historic survey of the colour question, it is 

 interesting to note that, like so many other things, it came to us 

 mainly by suggestion. These suggestions have been persistent 

 throughout the whole time that we have had any knowledge on 

 the subject. Many discoveries in photography have been brought 

 about by accident. The Daguerreotype plate was due to the 

 placing of an exposed plate by inadvertence near the vapour given 

 off by mercury. The discovery of the action of light on a silver 

 .salt was due to the use of nitric acid which had previously been 

 used to dissolve silver. The orthochromatising of plates was due 

 to an accident without which little progress would have been made 

 with colour photography. Pyrogallic acid for developing was also 

 an accidental discovery. But photography, colour photography, 

 and cinematography, through the desire to possess all these things, 

 have come to us by suggestion. Let me illustrate : In the year 

 1558 an Italian professor was showing to his friends a scientific 

 instrument which he called a camera obscura, literally a dark box. 

 In principle this was exactly the same as the camera we now carry 

 about in our pockets. In this instrument there was, first of all, 

 a picture of any object to which the lens was presented — a picture 

 written with pencils of light — which to-day w T e call photographic : 

 the picture was in colour — the pure colours of nature — bringing 

 us to colour photography : lastly, if there was any movement this 

 would be shown on the receiving surface, so giving us the 

 suggestion of cinematography. None of these things seem to have 

 suggested themselves to the professor's mind and centuries pass 

 before we hear of them again. To find any practical result from 

 these suggestions we must come down to about the year 1824. At 

 this time we find Daguerre unsuccessfully attempting to fix the 

 image seen in the camera obscura, which he used for sketching 

 purposes, being himself a scene painter and panoramist. Later 

 Niepce joined him in his researches, with the result that we all 

 know. So far w r e have got only as far as the discovery of photo- 

 graphy, though there is very little doubt that Niepce came very 

 near to the discovery of colour by the interference method. 



