;-52 Dr. R S. Clay. On the Application of 



the blue-green can be added mthmd being perceived even when the 

 diluted colour is compared directly with the pure (p. 132, 'Colour 

 Measurement ') It can thus easily be believed that much more can 

 be added without appreciable effect when no direct comparison is made 

 with the pure colours. 



But probably the best example of the unimportance of the addition 

 of white is given by Mr. Ives's results, where the colours used as 

 primary colours are produced by transmitting white through a red, 

 ^reen, and violet glass respectively; when, as in the examples above, 

 there is transmitted a light which is equivalent to a pure colour with 

 a percentage of white, which in some cases is quite considerable. I 

 confess my surprise at the excellence of the results obtained, which 

 show how exceedingly bad a judge the eye must be of an addition of 

 white to a colour. Provided the quantity that will be left in the 

 printing is not greater than the quantities he obtained by the trans- 

 mission through his glasses and I see no reason why inks should not 

 be produced to secure this the pictures obtainable by the three-colour 

 processes ought not to be inferior to those he obtains by the super- 

 position of the three-coloured transparencies. 



We can realise the effect of this addition of white by supposing a 

 coloured picture projected on a screen in a room that is not quite 

 dark. The screen will then reflect, in addition to the colours projected, 

 n certain amount of the diffused light of the room. As this coloured 

 picture would not consist of pure colours undiluted with white, the 

 addition of further white will make the proportion greater than would 

 ccur in printing. 



On the other hand, the eye is a very good judge of hue, a very 

 small variation in the proportion of the colours (other than white 

 being easily detected. 



Much may be done, by training, to educate the eye to appreciate 

 the relative luminosity of .the colour, or, as it is usually termed, the 

 " value " of the colour, and many artists are able to recognise varia- 

 tions in this almost as easily as in the hue itself. But the average 

 person is not a good judge of this, and it is of much less importance 

 than the hue. For instance, many people fail to see any improvement 

 in a photograph of a landscape or a bunch of flowers taken through an 

 orthochromatic screen, over a photograph of the same subject taken in 

 the usual way. 



But circumstances combine to make it difficult to recognise the 

 addition of white even in a picture : the irregularities in the varnishing 

 and the dust upon it are bound to add a proportion of white to the 

 light reflected, and indeed it is very seldom that a picture is so hung 

 that the varnish does not reflect a large amount of white, which we 

 often do not notice until we attempt to take a photograph of it, and 

 then very special lighting is found absolutely necessary. 



