452 



SUMMARY OF QUERENT RESEARCHES RELATING TO 



used as a mask to block out or weaken those colours which are not 

 wanted, the remainder coml)ining to form the picture. 



The surface composed of these contiguous narrow^ spectra is produced 

 by allowing white light to fall upon a very fine screen of nearly 400 

 lines per inch, in which the opaque lines are three times as wide as the 

 clear interspaces. An enlarged image of this screen is formed by means 

 of a lens with a prism just in front of it. The prism spreads each white 

 line into a complete spectrum, and is so calculated that the spectra lie 

 next to each other on the focusing screen without interspace. Thus, 

 on the focusing screen each complete spectrum is but ^-^ in. wide — 

 hence the term micro -spectra. If, instead of white light falling upon 

 the line screen, coloured light is caused to fall upon it, only those 

 spectrum colours of which the line in question is composed appear on 

 the focusing screen, the colours W'hich are wholly or partially missing 

 from the spectrum of white light being represented by spaces, wholly or 

 partially dark. 



In taking the photograph, the image of the coloured object is pro- 

 jected by means of an ordinary objective lens on to the line screen, the 



fOCUSSINC SCHZCn 



fHTTOaBAPWlC PlATt 



^ 



Fig. 82.— General optical arrangement shown diagrammatically. 



image of which is in turn projected by the second lens with the prism 

 in front of it on to the photographic plate placed in the position of the 

 focusing screen. 



The plate must be approximately equally sensitive to all colours, so 

 that the resulting negative is completely darkened when acted upon by 

 any colour in its full intensity and partially darkened where the incident 

 colour is weakened. A lantern slide positive from this negative will, of 

 course, show the reverse effect, being completely transparent where the 

 colour has acted with full intensity, of partial transparency where the 

 colour has acted less strongly, and opaque where the colours were 

 missing — i.e. in those parts "co-incident in position with the spectrum 

 colours of white hght that were not present in the object photographed. 

 When, therefore, the positive is placed in the exact position of the 

 negative, and w^hite light is projected through the apparatus, it acts as 

 the desired mask to bfock out those colours that are not wanted, and the 

 picture is reproduced in the original colours. 



An historical sketch shows that the general theory of the micro- 

 spectra method was suggested independently four times, once before and 

 twice after Rheinberg's suggestion, but the apparatus and the results 

 obtained by others were comparatively crude, there being very great 



