EEYIETTS. 
403 
who was familiar with the idea of an ether, and who introduced it in 
some of his speculations, objected, as already stated, that if light consisted 
of waves, shadows could not exist ; for that the waves would bend round 
the edges of opaque bodies and agitate the ether behind them. He was 
right in affirming that this bending ought to occur, but wrong in supposing 
that it does not occur. The bending is real, though in all ordinary cases it 
is masked by the action of interference. This inflection of the light receives 
the name of difiraction.” 
On all other branches of the important subject on which he has lectured 
the matter and manner of delivering it is the same, and we might chose 
many quotations if space permitted. But we must conclude with one more 
excerpt. It is upon the subject so often of late years illustrated by Dr. 
Tyndall, in his Institution lectures — the identity of heat and light in their 
mode of passage. “Perhaps,” says he, “no experiment more conclusively 
proves the substantial identity of light and radiant heat than the formation 
of invisible heat-images. Employing the mirror already used to raise the 
beam to its highest state of concentration, we obtain, as is well known, an 
inverted image of the carbon points formed by the light rays at the focus. 
Cutting off the light by the ray-fllter, and placing at the focus a thin sheet 
of platinized platinum, the invisible rays declare their presence and distribu- 
tion by stamping upon the platinum a white-hot image of the carbons.” A 
series of illustrations show at a glance the mode in which this interesting^ 
experiment is conducted. 
The authors remarks on Fraunhofer’s lines are also full of interest and 
instruction, but we cannot go into the subject. His concluding lecture is 
an eloquent peroration, and must have mightily charmed his audience. To 
the savant it seems to us that not the least valuable part of the work will 
be the quotations from Lord Brougham’s attack on Young, and Young’s able 
and masterly-written defence — of which we learn with sorrow, from his 
biographer’s sketch, that only a single copy was sold. Altogether we have 
been fascinated by this work of Dr. Tyndall’s, and we doubt not that its 
readers will be vast in numbers on this side of the Atlantic and the other. 
NICOL’S MINERALOGY.* 
H aving passed through a flrst edition, this book has now a second time 
come before the public, and in our opinion it is much improved in its 
new guise. Doubtless there are not a few who will object to the system of 
classiflcation adopted by the author, some fancying the crystallographical 
the only method, while others as strongly uphold the plan which is based 
on the chemical composition. But it seems to us that the author has 
adopted a wise course in his effort to steer between the two. And we fancy 
his mode of arranging minerals, while it is by no means complete, is 
“ Elements of Mineralogy,” containing a General Introduction to the 
Science, with Descriptions of the Species. By James Nicol, . F.R.S.E., 
F.G.S., Professor of Natural History in the University of Aberdeen. 2nd 
Edition. Edinburgh : Black, 1873. 
D D 2 
