Maxwell's Curves to Three-colour Work, &c. 



51 



The D line on the new scale was at 17 '7, and as it was also a milli- 

 metre scale, the new scale readings will all be 32*3 less than the 

 previous ones. 



To test this the following readings were taken : 



17. Determination of tlie Curves of Practical Inks. 



January 1, 1900. We set up the apparatus with the two right-angle 

 prisms and obtained the patches. We found some difficulty due to 

 stray light, which we nearly eliminated, so far as the outside of the 

 apparatus was concerned, by inclosing it with screens. A cloth was 

 placed over the arc and over the collimator and prisms, and a 

 blackened canvas screen was hung up to divide the end of the room 

 where the observations were made from the end containing the light 

 But when matching pigments in a part of the spectrum in which they 

 were nearly black, we were still able to perceive a small difference of 

 colour in the two patches, which was due to internal reflections on the 

 * Probably an error in reading. 



