506 
NATURE 
[Marcu 28, 1907 
Upon this is placed the microscope slide, with an 
intervening drop of cedar-wood oil, so that total re- 
flection does not occur again until the upper surface 
of the cover glass is reached, when the ray is again 
sent downwards and passes away through another 
bevelled edge. It will be understood that the pre- 
paration does not contain air. On this plan no 
immersion liquid can be employed in the usual place 
between the cover glass and the objective, but, on the 
other hand, the rays diffracted by small particles come 
off from the main beam at angles considerably smaller 
than a right angle. 
Several chapters of the book are devoted to the 
investigations which have been or can be carried out 
by these ultramicroscopes, of high interest to many. 
As examples, we may cite the distribution of silver, 
gold, and other metal particles in the coloured glasses 
containing them, and in the hydrosols of such metals; 
the Brownian movements of ultramicroscopic bodies 
in colloids, and the translation of such bodies by 
electric current. Especially interesting is the de- 
scription given of the motions of silver particles in the 
hydrosol of that metal prepared by the Bredig process 
of forming a submerged electric arc between silver 
wires. The particles, below certain dimensions, re- 
main in stable suspension. They are quite ultra- 
microscopic, but still are capable of diffracting light. 
When an electric current is passed through the liquid 
contained in a layer, not too thin, between top and 
bottom planes of glass, quartz, mica, &c., the micro- 
scope being focussed at the middle of the layer, at a 
point about equally removed from either electrode, the 
points of light seen move equably in a direction from 
the kathode to the anode, the speed being proportional 
to the potential gradient. For one volt per centimetre 
the speed is about 3-78 » per second. Above and 
below this central region, 7.e. in beds adjoining the 
top and bottom boundaries, the motion is in the 
opposite direction, somewhat slower and less equable, 
and variable with the size of the particles. 
If the boundary surfaces are of glass, these inverse 
beds are each about 25 » in depth, and if the thick- 
ness of the whole layer is diminished until it is only 
50 “, it is these inverse beds which survive, the 
central one being gradually extinguished. The 
motion will then be entirely from anode to kathode. 
The material of the boundaries affects the depth 
of the inverse beds, which with quartz is rather less 
than 2:5 “, and seems to disappear with gypsum. 
Mica has much the same effect as glass in this par- 
ticular. 
The particles have such exceedingly small mass 
that their ultimate velocities in the central region are 
acquired instantaneously, and if the electrodes are 
connected with an alternating source of electromotive 
force, the points of light move backwards and for- 
wards in harmony with the stress through a distance 
proportional to its mean value and to the period, the 
constant being sensibly consistent with the speed 
under uniform stress quoted above. If a three-phase 
machine is connected with three electrodes, the 
particles describe closed curves. 
Tuomas H. BLakeESLEY. 
NO. 1952, VOL. 75] 
ANCIENT AND MODERN SHIPS. 
Ancient and Modern Ships. By Sir George C. V. 
Holmes, K.C.V.O. Part i., Wooden Sailing-ships. 
Pp. xv+168. Part ii., The Era of Steam, Iron, 
and Steel. Pp. xii+219. (London: Printed for 
His Majesty’s Stationery Office by Wyman and 
Sons, 1906.) Two vols, cloth-bound, price 2s. 3d. 
each. 
HESE volumes belong to the series of science 
handbooks issued by the authorities of the 
Victoria and Albert Museum at South Kensington. 
The author was for a long period secretary of the 
Institution of Naval Architects; he is well qualified 
for the task he has undertaken. Within extremely 
narrow limits of space (about 4oo pages) he has pro- 
duced a readable account of ancient and modern 
ships, in which a large amount of trustworthy in- 
formation has been summarised and admirably illus- 
trated. Although the original intention of these 
handbooks may have been the assistance and in- 
struction of visitors to the collection of naval models 
in the museum, they will undoubtedly prove of interest 
as books of reference to all who are interested in the 
history of shipbuilding. Their moderate price ought 
to secure a large circulation. 
In the first volume wooden sailing-ships are de- 
scribed. This part of the work was published in 
tg00, but has been revised and re-issued in company 
with the larger second part, in which the history 
of the era of steam, iron, and steel is traced, so 
far as mercantile ships are concerned. War-ships, 
considered as fighting machines, are not dealt with, 
but the influence of peculiarities in their construction 
upon the development of mercantile shipbuilding is 
illustrated. Formerly, the naval models at South 
Kensington included those of war-ships; when the 
Royal School of Naval Architecture was transferred 
to Greenwich (more than thirty years ago) the 
Admiralty also concentrated there its collection of 
war-ship models. South Kensington retained the 
mercantile models, and the present collection includes 
loans from private firms, as well as models which 
are national property. It is much to be desired that 
the collection should be made complete and should 
illustrate adequately the development of the British 
mercantile marine. If Sir George Holmes’s hand- 
books should increase public interest in the collec- 
tion and lead to its proper development, a good pur- 
pose will have been served. At all events, he has 
produced a work which will enable laymen to reach 
an intelligent understanding of the history of ship- 
building and the principles governing the structural 
arrangements of ships. 
Beginning with an admirable account of ancient 
Egyptian vessels, the author describes boats still 
existing and to be seen in the Cairo Museum, 
although they were built nearly 5000 years ago. 
Ships of the Mediterranean and Red Seas— 
Phoenician, Greek, Roman, and Venetian—are next 
dealt with. Another chapter is devoted to the ancient 
ships of northern Europe, of which specimens have 
| been discovered in Scandinavia in recent years. 
