[Odtober, 
458 The Constants of Colour. 
Concerning the translation little need be said. Mr. Sand- 
bach thinks that if the book is allowed to be 100 or 150 years 
old, there is no reason for refusing it a greater antiquity. 
He thinks there is “ nothing in the narratives of this book 
inconsistent with probability,” and he endorses the absurd 
remark of Dr. Ottema that it is not more improbable for a 
“ clever woman to have become a lawgiver at Athens than 
for a goddess to spring, full-grown and armed, from the cleft 
skull of Jupiter ! ” Sancta simplicitas ! 
III. THE CONSTANTS OF COLOUR. 
By Professor 0 . N. Rood, of Columbia College. 
S HE tints produced by Nature and Art are so manifold, 
often so vague and indefinite, so affedted by their en- 
vironment, or by the illumination under which 
they are seen, that at first it might well appear as though 
nothing about them were constant ; as though they had no 
fixed properties which could be used in reducing them to 
order, and in arranging in a simple but vast series the 
immense multitude of which they consist. 
Let us examine the matter more closely. We have seen 
that when a single set of waves adts on the eye a colour- 
sensation is produced, which is perfedtly well defined, and 
which can be indicated with precision by referring it to 
some portion of the spedtrum. We have also found that 
when waves of light having all possible lengths adt on the 
eye simultaneously, the sensation of white is produced. 
Let us suppose that by the first method a definite colour- 
sensation is generated, and afterward by the second method 
the sensation of white is added to it ; white light is added 
to or mixed with coloured light. This mixture may be 
accomplished with an ordinary spedtroscope, by removing 
the scale from the scale-telescope, and replacing it by a 
vertical slit, as indicated in Fig. 1, which is a view from 
above. Then if white light be allowed to enter this slit, it 
will be reflected from the surface of the prism into the 
observing-telescope, and we shall find that the spectrum is 
crossed by a vertical band of white light. By moving with 
the hand the scale-telescope, this white band may be made 
to travel slowly over the whole spedtrum, and furnish us 
with a series of mixtures of white light with the various 
