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 of the film is considerably
darker than the rest of the spectrum; around this is a bright ring of