94 THE POPULAR SCIENCE MONTHLY. 



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 ex- 

 treme 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 percep- 

 tible 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 sup- 

 posed that there are no more rapid or slow rays, besides those already 

 known to us, and ranging in the number of vibrations from one hun- 

 dred 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 Monats- 

 hefte. 



-*- 



THE BOUNDARIES OF ASTRONOMY. 



i. 



IS GRAVITATION UNIVERSAL? 



Br KOBERT S. BALL, 



ASTBONOMER-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 promi- 

 nent 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 as- 

 tronomy which is often regarded as almost universally established. 



