42 



Dr. R S. Clay. On the Application of 



colours left in the reflected light also will vary with the depth. This 

 will not matter much when a large number of colours twelve or 

 fourteen are used, as the average amount of red and blue put on by 

 the successive inks will be nearly constant ; but with only one ink of 

 each colour the effect is very marked. 



This effect is greatest when the curve for the ink is a gradual one, 

 for as the quantity of ink is increased in an arithmetical progression, 

 the absorption of any given colour will increase in a geometrical pro- 

 gression, assuming, of course, that the ink is transparent. Thus, unless 

 the distributing apparatus of the machine is very perfect indeed, it will 

 be impossible with such an ink to obtain uniform impressions. 



Were the ink one with abrupt absorption, that is, one which Avas 

 very transparent up to a certain colour, and then nearly opaque, slight 

 variations in the quantity of ink would have very little effect, and one 

 of the greatest difficulties would at once disappear. 



12. Effi'd on tlie Purity of the Colour of an Abrupt Absolution. 



It might perhaps be supposed that inks which reflected most light in 

 the parts of the spectrum for which they are to be transparent, and 

 gradually reflect less up to the part they are to absorb, would be 

 nearer approximations to monochromatic inks than those whose absorp- 

 tions begin abruptly. That this is not the case can be easily seen if we 

 compare the two red-absorption inks opposite. In the second we have 

 replaced the reflected light ABD by BEG. Now Maxwell's curves 

 show that the proportion of red to green in BEG is much greater than 

 it is in ABD, so that the absorption of the red is not so complete in II as 



D 



it is in I, while the transparency to green is less. Thus II would not be 

 so good an ink as I. When, for instance, this and the violet absorption 

 ink are printed, which ought to leave green only, there would be some 

 red left, and this in greater amount than with the ink I. This would 



