32 Things not generally Known. 



In other words, light travels a distance equal to eight times 

 the circumference of the earth between two beats of a clock. 

 This is a prodigious velocity; but the measure of it is very cer- 

 tain. -Professor Airy. 



The navigator who has measured the earth's circuit by his 

 hourly progress, or the astronomer who has paced a degree of 

 the meridian, can alone form a clear idea of velocity, when we 

 tell him that light moves through a space equal to the circum- 

 ference of the earth in the eighth part of a second in the twink- 

 ling of an eye. 



Could an observer, placed in the centre of the earth, see this moving 

 light, as it describes the earth's circumference, it would appear a lumin- 

 ous ring ; that is, the impression of the light at the commencement of 

 its journey would continue on the retina till the light had completed its 

 circuit. Nay, since the impression of light continues longer than the 

 fourth part of a second, two luminous rings would be seen, provided the 

 light made two rounds of the earth, and in paths not coincident. 



APPARATUS FOR THE MEASUREMENT OF LIGHT. 



Humboldt enumerates the following different methods 

 adopted for the Measurement of Light : a comparison of the 

 shadows of artificial lights, differing in numbers and distance ; 

 diaphragms ; plane-glasses of different thickness and colour ; 

 artificial stars formed by reflection on glass spheres ; the juxta- 

 position of two seven-feet telescopes, separated by a distance 

 which the observer could pass in about a second ; reflecting in- 

 struments in which two stars can be simultaneously seen and 

 compared, when the telescope has been so adjusted that the 

 star gives two images of like intensity ; an apparatus having 

 (in front of the object-glass) a mirror arid diaphragms, whose 

 rotation is measured on a ring ; telescopes with divided ob- 

 ject-glasses, on either half of which the stellar light is received 

 through a prism ; astrometers, in which a prism reflects the 

 image of the moon or Jupiter, and concentrates it through a 

 lens at different distances into a star more or less bright. 

 Cosmos ', vol. iii. 



HOW FIZEAU MEASURED THE VELOCITY OF LIGHT. 



This distinguished physicist has submitted the Velocity of 

 Light to terrestrial measurement by means of an ingeniously 

 constructed apparatus, in which artificial light (resembling 

 stellar light), generated from oxygen and hydrogen, is made 

 to pas? back, by means of a mirror, over a distance of 28,321 

 feet to the same point from which it emanated. A disc, hav- 

 ing 720 teeth, which made 12'6 rotations in a second, alter- 

 nately obscured the ray of light and allowed it to be seen 

 between the teeth on the margin. It was supposed, from the 

 marking of a counter, that the artificial light traversed 56,642 



