GHT
HIS MISTAKE AS REGARDS ACHROMATISM
SYNTHESIS OF WHITE LIGHT
YELLOW AND BLUE LIGHTS PRODUCE WHITE BY THEIR MIXTURE
COLOURS OF NATURAL BODIES
ABSORPTION
MIXTURE OF PIGMENTS CONTRASTED WITH MIXTURE OF LIGHTS.

Sec. 1. _Introduction_.

Some twelve years ago I published, in England, a little book entitled
the 'Glaciers of the Alps,' and, a couple of years subsequently, a
second book, entitled 'Heat a Mode of Motion.' These volumes were
followed by others, written with equal plainness, and with a similar
aim, that aim being to develop and deepen sympathy between science and
the world outside of science. I agreed with thoughtful men[1] who
deemed it good for neither world to be isolated from the other, or
unsympathetic towards the other, and, to lessen this isolation, at
least in one department of science, I swerved, for a time, from those
original researches which have been the real pursuit and pleasure of
my life.

The works here referred to were, for the most part, republished by the
Messrs. Appleton of New York,[2] under the auspices of a man who is
untiring in his efforts to diffuse sound scientific knowledge among
the people of the United States; whose energy, ability, and
single-mindedness, in the prosecution of an arduous task, have won for
him the sympathy and support of many of us in 'the old country.' I
allude to Professor Youmans. Quite as rapidly as in England, the aim
of these works was understood and appreciated in the United States,
and they brought me from this side of the Atlantic innumerable
evidences of good-will. Year after year invitations reached me[3] to
visit America, and last year (1871) I was honoured with a request so
cordial, signed by five-and-twenty names, so distinguished in science,
in literature, and in administrative position, that I at once resolved
to respond to it by braving not only the disquieting oscillations of
the Atlantic, but the far more disquieting ordeal of appearing in
person before the people of the United States.

This invitation, conveyed to me by my accomplished friend Professor
Lesley, of Philadelphia, and preceded by a letter of the same purport
from your scientific Nestor, the celebrated Joseph Henry, of
Washington, desired that I should lecture in some of the principal
cities of the Union. This I agreed to do, though much in the dark as
to a suitable subject. In answer to my inquiries, however, I was given
to understand that a course of lectures