34 



Dr. R S. Clay. On the Application of 



ordinary commercial printing to control the extent to which they 

 overlap; that is to say, the successive impressions will, owing to 

 minute differences in " registering " the paper, have the dots of the 

 different colours more or less displaced in relation to each other. 



If, for instance, the blue dots and the pink dots happened each to 

 exactly cover half the area, in one impression they might be exactly 

 superimposed, and in another they might hardly overlap at all. In the 

 former case half the area will be printed with both pink and blue, and 

 half will be white. In the latter case half the area is printed with blue 

 and half with pink, and there is no white left. If these impressions 

 are to be equally good, the resulting absorptions through the spectrum 

 .should be the same in the two cases. 



Let the adjoining curves represent the percentage transparency of 

 the inks. At A both inks are perfectly transparent, and as the colour 

 is not absorbed by either, it can of course make no difference' to this 

 colour if the dots are superimposed or not. 



(too 



100 



At B each colour absorbs half the light and reflects half the light. 

 When the dots are printed adjacent to one another, half the total light 

 of that colour will be reflected. When they coincide, half the paper is 



left white : this reflects all the light it receives, which is half the total. 

 The blue ink on the other half absorbs half the light which it 



