362 PROCEEDINGS OF THE AMERICAN ACADEMY. 



of intensity, the greatest intensity possible to the pigments on our pal- 

 ette. This being done, we shall observe, at once, that the colors have a 

 natural order or connection with one another. Red passes into green 

 through yellow ; yellow passes into blue through green ; and blue 

 passes into red through violet. There is, in other words, a natural 

 relationship for all the colors we can produce in the same value and 

 intensity. There is a natural scale of colors, as there was a natural 

 scale of values. This scale of colors is, of course, the scale of the 

 spectrum. The spectrum which I have followed in this investigation, is 

 the normal spectrum, the spectrum of the grating, not the spectrum 

 of the prism. The difference is explained by Rood, by Lommel, and 

 by other writers. Reading the spectrum from the red end towards the 

 violet end, the colors follow one another, approximately at equal inter- 

 vals of equal contrasts, as follows : red, suggesting Chinese vermilion ; 

 yellow, suggesting aureolin ; green, suggesting emerald green ; blue, sug- 

 gesting cobalt with a little emerald green in it; violet, suggesting ultra- 

 marine with a little rose madder in it. Beyond the violet end of the 

 spectrum we may observe the color which we call purple. It suggests 

 rose madder witli a little ultramarine in it. This color does not belong 

 in the spectrum series. It is due to the overlapping of the red and 

 violet ends of the spectrum. It is, however, a color which we must use, 

 and if the primary or important colors of the spectrum are red, green, 

 and violet, purple exists for us as an intermediate between red and 

 violet, just as yellow is intermediate between red and green, and blue 

 between green and violet. Between the six colors, red, yellow, green, 

 blue, violet, and purple, come intermediates and the intermediates of 

 intermediates up to the limit of visual discrimination. Setting the nor- 

 mal spectrum upon the circumference of a circle, with purple as a con- 

 necting link between the ends, the interval between any two colors can 

 be described as an interval of so many degrees. We have an interval 

 of 60°, the interval separating red from yellow, yellow from green, green 

 from blue, blue from violet, violet from purple, adjacents in the scab' of 

 six colors. We have the interval of 120° between the adjacents of a 

 scale of three colors, — red, green, and violet, I'm- example; and we have 

 the interval of 180° between the adjacents in the scale of two colors, — 

 red and blue, for example. This is the greatest possible interval. Ir is 

 the interval between colors opposite one another in the circle, the colors 

 which we call complementaries. In order to study the various intervals 

 of the BCale of colors with certain conclusions, we must eliminate all 

 differences of value and all differences of intensity. Wc can then 



