SCIENCE-GOSSIP. 



Now Du Hauron took his negatives through red, 

 green, and blue filters, but in making his triple 

 superimposed prints he did not print from the red- 

 filter negatives in red ink, but in a greenish-blue 

 colour ; similarly he printed from the green-filter 

 negative in a pink colour, and from the blue-filter 

 negative in avellow colour. A little consideration will 



nearest the eye from that passed by the other print. 



Think for one moment what we do when we paint, 

 say, a streak of blue paint on a piece of white 

 card : the white card is reflecting light of all 

 colours to the eye, the streak of blue paint absorbs 

 a part of the white light, the red and green rays, 

 and makes the card appear darker, because the 



w 





R 





G 





B 





Y 





Blk 



Test object representing 



. squares of white, red, green, 



blue and yellow -' 



and black. 



Negative of above test ob- 

 B ject taken through the red 



iieht filter. 



Negative taken through 

 the green lisht filter. 



Negative taken through 

 the blue-violet light filter. 



1 



Print from the negative 

 taken through the red light 

 filter, printed in cyan blue 

 pigment transmitting 

 green and blue-violet light 

 or minus red. 



Print from the negative 

 takeii through the green 

 light filter, printed in pink 

 pigment transmitting red 

 and blue-violet or minus 

 green. 



Print from the negative 

 taken through the blue- 

 violet light filter, printed 

 in yellow pigment trans- 

 mitting red ;\\v\ green light 

 or minus blue. 



tt The result of superimpos- 

 ing prints E, F, and G. 



show us the reason of this change. In the experi- 

 mentof the three overlapping discsof red, green, and 

 blue light shown in fig. 7 we mixed all three lights 

 and produced white light ; when, however, we super- 

 pose coloured prints we are not adding one 

 coloured light to another, but rather adding dark- 

 ness, abstracting the light passed by the print 



paint only reflects the blue rays to the eye. There- 

 fore, in printing upon paper or in superposing trans- 

 parent prints for lantern slides, we print, not in the 

 primary colours red, green, and blue, but in their 

 complementaries. The red-filter negative we print 

 in a colour transmitting the complementary of 

 red — i.e. minus red. that colour which with red 



