We introduce a very thin-walled glass cell, filled with such a solution, between the reflectors. Light is now cut off, but heat passes through freely. The focus is absolutely dark, but it still contains heat, of which fact we can soon convince ourselves by introducing a cigar into it: it is ignited and bursts into flame. White as well as black paper is charred and ignites in it. A piece of platinum-foil is raised to white heat in the dark focus. If we examine the incandescent platinum with a prism, we find that it emits again all colors, from the most extreme red to the most extreme violet; consequently, we have here the counterpart of fluorescence. The dark rays, by the augmentation of the number of vibrations, are converted into luminous ones, influencing the eye. Tyndall, who first observed and examined this appearance, called the conversion calorescence.
We have thus passed through a domain of physics, the more exact knowledge of which we mainly owe to the researches of our century, more especially to very recent times. That part of radiation perceptible to our organs of sense was extended far beyond the violet end of the spectrum, in investigating the chemical effects of light and fluorescence. We succeeded at the same time in rendering visible that part simply felt by the eye. It can not for a moment be supposed that there are no more rapid or slow rays, besides those already known to us, and ranging in the number of vibrations from one hundred and sixty to two thousand billions. Their existence can just as little be doubted as that of the ultra-violet. Whether we shall ever succeed in rendering them perceptible to our organs of sense remains a task for the investigations of the future.—Westermann's Monatshefte.
IS GRAVITATION UNIVERSAL?
By ROBERT S. BALL,
ASTRONOMER-ROYAL OF IRELAND.
IT is proposed in this and the following paper to trace some parts of the boundary-line which divides the truths which have been established in astronomy from those parts of the science which must be regarded as more or less hypothetical. It will be obvious that only a small part of so wide a subject can be discussed, or even alluded to, in the limits proposed. We intend, therefore, to select certain prominent questions, and to discuss those questions with such fullness as the circumstances will admit.
It will be desirable to commence with that great doctrine in astronomy which is often regarded as almost universally established.