pinion of Mr. De Moltke himself that the German siege batteries
would have been reduced to silence, had the defenders been able to hold out
for a week longer. It is equally certain that, during the course of the
Loire campaign, eighty guns of Prince Frederick Charles' were put out of
service by the sole fact of their firing. Summing up the history of these
many accidents, the Duke of Cambridge asserted to the House of Lords (April
30, 1876) that _two hundred_ Krupp guns burst during the Franco-German war.
Have the engineers of the Essen works improved their processes of
manufacture since that epoch? It is permissible to doubt it, seeing that,
very recently, the Italian navy refused to take from Mr. Krupp some 151/2
inch guns whose tubes were but very imperfectly welded.

Must the numerous accidents mentioned be attributed to defects in the metal
employed? Were they due to defective hooping? Were they due to some one of
the numerous inconveniences inherent to the cylindrico-prismatic system of
closing (_Rundkeilverschluss_)?

They were doubtless owing to such causes combined.--_La Nature_.

* * * * *

COLORS OF THIN PLATES.

The Right Hon. Lord Rayleigh lately delivered a lecture at the Royal
Institution upon "The Colors of Thin Plates," a term which he explained was
applied to thin films of substances, such as oily films on the surface of
water or the equally familiar soap bubble. Although the reflection of
colors from the surface of a soap bubble is probably the most noticeable,
yet the "plate" which lends itself most readily for experiment is a film of
air confined between two sheets of glass. If a ray of white light be
reflected from the surface of the film upon a screen, the so-called
Newton's rings, a series of colored concentric rings, are obtained. If,
instead of reflected light, the ray of light transmitted through the film
of air be allowed to fall upon the screen, the same phenomenon is
observable, but the effect is very considerably minimized, owing to the
great preponderance of white light, which overlies as it were the colored
rings. Even in the first instance, as the lecturer was able to show later
on, the colors are not nearly so intense as they may be obtained, owing to
some white light being reflected from the surfaces of the two sheets of
glass. With regard to the appearance of the phenomenon, it is observed that
the part which corresponds to the thinnest part