NATURAL miLOSOPIlY. 1G7 



PHENOMENA PRODUCED BY REVOLVING DISCS. 



Prof. Dove, some years ago, obtained a lustrous appearance by the 

 binocular combination of geometrical figures executed in black and 

 white, or in complementary colors ; and in 1861 Prof. I). Rood showed 

 that surfaces without drawings produced the same effect, publishing 

 his experiments in Silliman's Journal . The latter gentleman, in the 

 .same journal, has just published some additional experiments from 

 which we extract the following: "A circular disc of white card- 

 board, nine inches in diameter, with half its surface painted of a dead 

 black, was caused to rotate by clock-work at varying rates, while 

 the bright light from a window fell upon it. A stereoscope, from 

 which the ground glass had been removed, was provided with a 

 card-board in which were cut two square apertures, at such a distance 

 asunder that their binocular union could be easily effected, and while 

 the disc was at rest the stereoscope was arranged so that through the 

 right hand aperture some of the white portion of the disc was seen, 

 and through the left-hand aperture a part of the blackened surface. 

 On communicating rotary motion to the discs, a more or less rapid 

 alternation of black and white was the result. It was found that with 

 slow rates of rotation the strength of the luster was not impaired, 

 and it was just as plainly perceptible with more rapid rates. But 

 when the disc was made to revolve so fast that its surface seemed cov- 

 ered by a uniform tint of gray, and the so-called flickering had 

 ceased, no luster, in the proper sense of the term, could be seen, the 

 appearance being exactly that which is presented to a single eye under 

 similar circumstances." 



ON THE INVISIBILITY OF NEBULOUS MATTER. 



The following communication on the above subject has been made 

 to Silliman's Journal, by D. Trowbridge, Esq. 



It has generally been supposed that if nebulous matter, in the proper 

 sense of the word, or cosmical vapor, exists in the heavens, and within 

 reach of our telescopes, it will be visible to the eye with suitable opti- 

 cal aid. It is proposed to show in this article, with some plausibility, 

 that this is an erroneous idea, except in some particular cases. 



Comets are the only celestial objects, whose physical constitution 

 is approximately understood, that afford us anything like a distinct 

 intion of what nebulous matter is. By far the greater proportion of 

 these bodies are composed of materials so extremely rare that the 

 solar rays can penetrate completely through the denser portion of their 

 bodies, and the light in some cases seems to suffer scarcely any dim- 

 inution in intensity. Yet these bodies, which perhaps would weigh 

 at the surface of the earth but a few ounces, or but a few pounds, are 

 distin.ctly visible with the smallest optical aid, and even, under favor- 

 able circumstances, with the naked eye. Sir John Ilerschel says, of 

 this class of comets, that the most unsubstantial clouds which float in 

 the higher regions of our atmosphere, must be looked upon as dense 

 and massive bodies in comparison with the almost spiritual texture of 

 these light bodies. A cloud composed of materials so rare, and whose 

 distance from us did not exceed tilteen or twenty miles, would scarce- 



