Curiosities of Science. 33 



feet, or the distance to and from the stations, in -j-g-Voth part of 

 a second, whence we obtain a velocity of 191,460 miles in a se- 

 cond.* This result approximates most closely to Delarnbre's 

 (which was 189,173 miles), as obtained from Jupiter's satellites. 



The invention of the rotating mirror is due to Wheatstone, who made 

 an experiment with it to determine the velocity of the propagation of the 

 discharge of a Leyden battery. The most striking application of the 

 idea was made by Fizeau and Foucault, in 1853, in carrying out a pro- 

 position made by Arago, soon after the invention of the mirror : we have 

 here determined in a distance of twelve feet no less than the velocity 

 with which light is propagated, which is known to be nearly 200,000 

 miles a second; the distance mentioned corresponds therefore to the 

 77-millionth part of a second. The object of these measurements was 

 to compare the velocity of light in air with its velocity in water; which, 

 when the length is greater, is not sufficiently transparent. The most 

 complete optical and mechanical aids are here necessary: the mirror of 

 Foucault made from 600 to 800 revolutions in a second, while that of 

 Fizeau performed 1200 to 1500 in the same time. Prof. Helmholtz on 

 the Methods of Measuring very small Portions of Time. 



WHAT IS DONE BY POLAEISATION OF LIGHT. 



Malus, in 1808, was led by a casual observation of the light 

 of the setting sun, reflected from the windows of the Palais de 

 Luxembourg, at Paris, to investigate more thoroughly the phe- 

 nomena of double refraction, of ordinary and of chromatic po- 

 larisation, of interference and of diffraction of light. Among 

 his results may be reckoned the means of distinguishing between 

 direct and reflected light ; the power of penetrating, as it were, 

 into the constitution of the body of the sun and of its luminous 

 envelopes ; of measuring the pressure of atmospheric strata, 

 and even the smallest amount of water they contain ; of ascer- 

 taining the depths of the ocean and its rocks by means of a 

 tourmaline plate ; and in accordance with Newton's prediction, 

 of comparing the chemical composition of several substances 

 with their optical effects. 



Arago, in a letter to Humboldt, states that by the aid of his polari- 

 scope, he discovered, before 1820, that the light of all terrestrial objects 

 in a state of incandescence, whether they be solid or liquid, is natural, 

 so long as it emanates from the object in perpendicular rays. On the 

 other hand, if such light emanate at an acute angle, it presents mani- 

 fest proofs of polarisation. This led M. Arago to the remarkable co:i- 

 clusion, that light is not generated on the surface of bodies only, but that 

 some portion is actually engendered within the substance itself, even in 

 the case of platinum. 



A ray of light which reaches onr eyes after traversing many 

 millions of miles, from the remotest regions of heaven, an- 

 nounces, as it were of itself, in the polariscope, whether it is 



* Fizeau gives his result in leagues, reckoning twenty-five to the equatorial 

 degree. He estimates the velocity of light at 70,000 such leagues, or about 

 210,000 miles in the second. 



