SCIENCE-GOSSIP. 



iog 



and a blue-violet, and place them in three optical 

 lanterns, and so arrange the optical lanterns thai 

 the projected discs may overlap, as shown in fig. 7. 

 we shall find that where the red disc overlaps the 

 creen we get yellow light, where the green disc 

 overlaps the blue we get greenish-blue light, and 

 where the blue disc overlaps the red we get pink 

 light, whilst where all three overlap in the centre 



C D 



lijM. 1 



E *--: 



Li jM I 



Fig. 5. {Vide p. 67.) 



we get white light. By no possible combination 

 of coloured lights can we produce a pure red, 

 green, or blue, so we are forced to accept, these 

 colours as the primaries. If, however, we examine 

 the reconstituted white light by means of a spectro- 

 scope, we shall find that instead of getting a con- 

 tinuous spectrum, as we do, for instance, with the 

 light reflected from a white cloud, we only get 

 three narrow bands of colour — red, green, and 

 blue-violet ; but both the light from the white 



rt. 



J 



w-< 



Omriqc 



b itcUou- 



cloud and the reconstituted white light appear 

 alike to the eye. 



In view of these facts, it would appear very 

 simple to take three photographs of a coloured 

 object, one through a filter admitting red light, one 

 through a filter admitting green light, and one 

 through a filter admitting blue-violet light, and, by 

 projecting prints from these negatives illuminated 

 by red, green, and blue lights upon a screen in 



superpositions, secure a reproduction of the object 

 photographed in its natural colours. The first 

 photographs so produced were shown in public by 

 Professor Clerk-Maxwell in 1861. InMayofthal 

 year Clerk-Maxwell delivered a lecture at bhe 



Royal Institution, from which the following is 

 extracted : — 



"Experiments on the prismatic spectrum show 

 that all the colours of the spectrum and all the 

 colours in nature are equivalent to mixtures of 

 three colours of the spectrum itself, namely, red, 

 green (near the line E ), and blue (near the line G). 



" The speaker, assuming red, green, and blue as 



primary colours, then exhibited them on a - 

 by means of three magic-lanterns, before which 

 were three glass troughs containing, respectively, 

 sulpho-cyanide of iron, chloride of copper, and 

 ammoniated copper. 



