136 O. N. Rood—Indigo in the Spectrum. 
The Winsor and Newton disc which the previous experi- 
ment had proved to be the least greenish in hue, was now com- 
bined with one of vermilion and emerald green, and the fol- 
lowing equation obtained : 
I 514+ V 29+G 19°6 =32°8 white. A disc of Prussian blue 
similarly treated gave P.b. 89:°9+V 85°7+G 24-4 =27-4 white. 
ese equations prove that the hue of the indigo and Prus- 
sian blue discs were identical, for the ratio of the red and green 
required to effect neutralization is the same, being in the case 
of the indigo, 59°7 vermilion to 40°83 emerald green; in that of 
the Prussian blue, 59-4 vermilion to 40°6 emerald green. 
The position of the Prussian blue disc in the normal spec- 
trum was now determined with the aid of a large spectrometer, 
the eye-piece being provided with a slit which excluded all 
except a narrow slice of the spectrum. Such determinations 
can be made by a practiced eye with considerable certainty, as 
I propose to show at some future time. It was found that in 
a normal spectrum including from A to H 1000 parts the posi- 
tion of Prussian blue was at a distance from A equal to 740 of 
these parts. Now according to my observations on this spec- 
trum, blue-green ends and cyan-blue begins at 698; also cyan- 
blue ends and blue begins at 749; hence the color of Prussian 
blue falls in the cyan-blue space near the beginning of the blue, 
and to this same position we must consequently refer the color 
of indigo. 
It afterwards occurred to me that possibly Newton might 
have used the indigo in the dry lump, and accordingly I pre- 
pared a flat surface of dry commercial indigo and compared it 
carefully with the blue furnished by genuine and artificial 
ultramarine, its color being of course enormously darker, or 
one might say, blacker than that of either of these substances. 
A mixture by rotation of six parts of artificial ultramarine blue 
with two parts white and ninety-two parts black gives a color 
more or less like that of commercial indigo in the dry cake: 
that is to say, if a freshly fractured surface of indigo be com- 
pared with the compound disc just mentioned, the color of the 
indigo will be found somewhat too greenish; but on the other 
hand, if a scraped surface of the dry cake is used it will be too 
purplish. Newton therefore probably employed his indigo in 
the dry state. 
I give below, according to my determinations, the positions 
and corresponding wave-lengths of indigo, Prussian blue, cobalt- 
Jue, genuine ultramarine-blue and artificial ultramarine-blue, 
in a normal spectrum having from A to H 1000 parts. 
