100 Notices of Books. (January, 
admirably clear explanation which is given of the cause of 
refraction by a prism, and of the change of wave-front. The 
polarised beam of light is compared, in the matter of its two- 
sidedness with the two-endedness of a magnet. The subject of 
polarisation, and the chromatic phenomena which accompany it, 
is continued in the fourth lecture; and the many admirable 
experimental illustrations with which Dr. Tyndall has made us 
familiar at the Royal Institution are freely introduced to develope 
the subject more clearly. 
The fifth lecture takes us into Dr. Tyndall's more special 
domain of radiant heat, and the consideration of its relationship 
to light. The subject of Fluorescence is fully discussed, and 
the usual solution of sulphate of quinine, and the recently 
discovered Thallene of Prof. Morton used to illustrate it. Then 
we have an interesting paragraph relating to what is now called 
‘Potential Light.” ‘‘ Fluor-spar,and some other substances, when 
raised to a temperature still under redness, emit light. During 
the ages which have elapsed since their formation, this capacity 
of shaking the ether into visual tremors appears to have been 
enjoyed by these substances. Light has been potential within 
them all this time; and, as well explained by Draper, the heat, 
though not itself of visual intensity, can unlock the molecules so 
as to enable them to exert the power of vibration which they 
possess.” The observations of Dr. Bence Jones appear to have 
proved the existence of a fluorescent substance in the human 
body, notably in the lens of the eye. Thus, if the eye be 
plunged into the ultra-violet rays, it becomes conscious of a 
‘‘whitish-blue shimmer” filling the space before it; and, ac- 
cording to Dr. Tyndall, the crystalline lens of the eye, if it be 
then viewed from without, is seen to gleam vividly. An account 
of calorescence, and of the effects of the non-luminous ultra- 
red rays is next given, and, near the end of the lecture, of the 
refraction, polarisation, and magnetisation of heat. We cannot 
completely understand the connection between the ultra-red rays 
and certain phenomena in Nature (described on pp. 176 and 177, 
and could almost imagine that a page or paragraph had been 
omitted. We can find nothing in the text to prove that the 
invisible ultra-red rays produce the warming and consequent 
evaporation of the tropical oceans, and hence, indirectly, our 
rains and snows. Again, a large flask containing a freezing 
mixture, and coated with hoar-frost, was placed at an intensely 
luminous focus of electric light, an alum-cell being interposed ; 
the frost was not melted: when, however, an iodine-cell, which 
cut off all the light, was interposed, and the alum-cell withdrawn, 
the hoar-frost was melted. ‘Hence,’ says our author, ‘‘ we 
infer that the snow and ice which feed the Rhone, the Rhine, and 
other rivers which have glaciers for their sources, are released 
from their imprisonment upon the mountains by the invisible 
ultra-red rays of the sun.” Why hence? The experiment 
