; 
Nov. 23, 1882] 
NATORE 
a3 
Elpidia glacialis, has been dredged at this inconsiderable 
depth, and even this was dredged in the Arctic Ocean, 
where true abyssal forms are to be met with at compara- 
tively shallow depths. This species too can exist at 
immense depths, one from Station 160 having been 
dredged at a depth of 2600 fathoms, the greatest depth 
at which any Holothuroid has to this been dredged being 
2900 fathoms. Among the more remarkable and dis- 
tinguishing characteristics of this order Mr. Théel men- 
tions the agreement in several important details—both in 
their internal anatomy and outer forms—of the adult and 
larval forms, an agreement more close than occurs in any 
previously known Holothuroid. He does not agree with 
Danielssen and Korren in placing the Elasipods low in 
the series of the Holothuroids ; nay in some respects he 
regards them as having attained to a higher development 
than all the other Echinoderms, because, among other 
facts, their bodies are distinctly bilaterally symmetrical, 
with the dorsal and ventral surfaces distinct and often 
with a cephalic region well marked. Only the ventral 
ambulacre are subservient to locomotion; these latter 
show a tendency to appear both definite as to place and 
number. The dorsal appendages are so modified as to 
perform functions different from the ventral ones. This 
memoir contains forty-six plates, which give full details 
of the forms and structure of all the new species. 
LIGHT 
Light: A Course of Experimental Optics chiefly with the 
Lantern. By Lewis Wright. (London: Macmillan 
and Co., 1882.) 
HIS is a book by a worker whose work in his own 
line is of a very high order, and whose experience 
will be of correspondingly high value to others who are 
working at the same subject. In all those departments 
of experimental optics in which the lantern is employed 
for the demonstration of actual experiments to an audi- 
ence, Mr. Wright is a master hand: and his book, as 
might be expected, is consequently a valuable repertory 
of useful information and of suggestive hints. Of books 
on Light there are already enough and to spare. Of 
standard treatises and text-books in the department of 
Geometrical Optics the supply is more than could be 
desired. In Physical Optics there is still room for a good 
elementary mathematical text-book. In Physiological 
Optics also there is, save for the great treatise of Helm- 
holtz, a void. But the work before us stands apart from 
all these, both in aim and in character. Indeed so well 
does it carry out the ideal of a work “on experimental 
optics chiefly with the lantern,’’ that there was really no 
need to prefix to the title the word ‘‘ Light.” True it is 
that Mr. Wright does not confine himself to the mere 
working of lanterns and their accessories. He deals ina 
simple and practical way with the laws of reflexion and 
refraction, and with ordinary optical instruments : but he 
always adds something of practical interest to the teacher 
of optics. To illustrate the laws of reflexion and refrac- 
tion he describes a simplified form of the apparatus so 
well known in Prof. Tyndall’s lectures on Light; and the 
mechanical illustrations of wave-motion, &c., are also 
new in several respects. The chapter on Spectrum 
Analysis is brief and sketchy, but includes almost all the 
experiments which can be projected on to the screen with 
the lantern. Amongst these we notice very careful in- 
structions for exhibiting the spectrum of Newton’s rings 
and of other interference phenomena, 
Nearly one-half of the book is devoted, and well 
devoted, to experimental work on Double-Refraction and 
Polarisation. In this section there are a number of 
beautiful experiments described which we do not remem- 
ber having seen before in any treatise in the English 
language. Amongst these are some with compound mica 
plates built up of a series of films of definite thickness 
and united by Canada balsam. A series of twenty-four 
superposed mica films, each producing a retardation of 
one-eighth of a wave-length and each one-sixteenth of an 
inch shorter than the one beneath it, is in this way made 
to reproduce exactly the first three orders of colours of 
Newton’s rings, but divided into the precise tints over 
narrow strips. A detailed account is also given of the 
combinations devised by Norremberg and Reusch for 
reproducing the phenomena of uniaxial crystals and of 
quartz by the superposition of thin films of mica crossed 
in various ways. Plates illustrative of these combinations 
contribute much to the value of the descriptions and 
explanations of the text. Mr. Wright also gives some 
account of his own researches upon the spiral figures 
produced by the introduction of quarter-undulation plates 
into the polariscope in which crystal sections are being 
examined by convergent light. There is a penultimate 
chapter on the polarisation of the sky and of minute par- 
ticles, followed by a final chapter—wholly out of place in 
such a work—in which, so far as it is intelligible, there 
appears to be an attempt made to connect the undulatory 
theory of light with the trinitarian theory of theology. 
With the exception of this last, and with a few occasiona 
inelegancies of style, there is little fault to find with the 
book. The mathematical student of optics will without 
doubt grumble when he takes up the work, because the 
mathematical aspect of the subject is conspicuous by its 
absence. The author does not profess to be a mathe- 
matician : or he would hardly have pronounced in favour 
of Brewster’s views on the theoretical polarising angle, 
as he does on p. 223. This, however, is a minor matter 
in a book whose great aim is to assist manipulation. 
The numerous illustrations, a large proportion of which 
are original, add greatly to its value. The coloured 
plates of polariscopic phenomena are, it should be added, 
of singular excellence. Sobol 
OUR BOOK SHELF 
Practical Chemistry, Analytical Tables, &c. By J 
Campbell Brown, D.Sc. (London: Churchill, 1882.) 
NOTHING perhaps is more remarkable than the great 
increase during the past few years in the number of books 
on practical chemistry and analysis. This has no doubt 
to some extent been caused by the prominence given 
generally to the teaching of chemistry in the laboratory. 
The books to which we refer consist with few excep- 
tions of tabular statements of reactions of acids and bases 
and methods of detection of the same in simple salts or 
mixtures. They all appear to be on the same “type” 
and with the same intention of putting students through 
a course of drudgery in qualitative analysis according to 
a fixed “table.” The book before us is no worse than 
others of its class, but attempts rather too much by giving 
