98 Notices of Books. [January, 
NOTICES OF .BOOES: 
Lectures on Light. Delivered in America in 1872-1873. By 
Joun Tynpatt, LL.D., F.R.S., Professor of Natural 
Philosophy in the Royal Institution. London: Longmans, 
Green, and Co. 1873. 8vo., 268 pp. 
WE are all more or less familiar with the history of Dr. Tyndall’s 
recent visit to America. We know that he was invited to that 
country to give a course of lectures on Natural Philosophy in 
the principal cities of the States, and that he was received 
everywhere with open arms. The thirst of the Americans for 
science is prodigious ; a considerable scientific taste and literature 
is springing up amongst them; their enterprise induces them to 
constantly reprint our large works on science; and it will be 
remembered that these very lectures of Dr. Tyndall’s were printed 
in broadsides, illustrated, and issued, in a newspaper-like form, 
at the cost of afewcents. The subject chosen was Light, and, 
in the work before us, we have the course of six lectures on that 
subject, which Dr. Tyndall delivered in the principle centres of 
American thought and progress. 
Dr. Tyndall has adopted the plan of giving a history of the 
Science of Light, and illustrating each fact as it was discovered. 
Thus early in the first lecture we find an account of the law, that 
the angle of incidence is equal to the angle of reflection; of 
Snell’s Law of the Refraction of Light (1621), sometimes called 
‘Descartes’ Law;’ and of Rcemer’s Determination of the 
Velocity of Light (1676). ‘Snell’s Law of Refracton,” says our 
author, ‘“‘is one of the corner stones of optical science, and 
its applications to-day are millionfold. Immediately after its 
discovery, Descartes applied it to the explanation of the rainbow. 
A beam of solar light falling obliquely upon a raindrop is 
refracted on entering the drop, and, on emerging, is again 
refracted.” Here follows an explanation of the rainbow, and 
the means by which Descartes proved its origin. Next we have 
an account of Newton’s Discovery of the Decomposition and 
Recomposition of Light, and many illustrations of the discovery ; 
then of Achromatism, and the Theory of Colours. 
In the second lecture we are introduced to the once rival 
conjectures concerning the nature of light:—The ‘* Emission 
Theory,” and the ‘‘ Undulatory Theory ;” and here Dr. Tyndall 
introduces some very pertinent remarks regarding the conception 
of a physical theory, and the necessary use of the imagination 
in that form of conception :—‘‘ This conception of physical theory 
implies, as you perceive, the exercise of the imagination. Do 
not be afraid of this word, which seems to render so many 
respectable people, both in the ranks of science and out of them, 
