128 ANNUAL EECOKD OF SCIENCE AND INDUSTRY. 



gel of the spectrum of the lightning discharge, quoted by 

 Professor Rood, some flashes give spectra of bright lines on 

 a dark background, while others give bright lines on a less 

 bright continuous spectrum as a background, and, finally, 

 some give a continuous spectrum destitute of lines. Rood 

 thinks it probable that the continuous spectrum corresponds 

 to the prolonged constituents of the flash, and tliat the nor- 

 mal spectra of bright lines on a dark ground were produced 

 by flashes more nearly instantaneous. 4 -Z>, March ^ 1873. 



APPARATUS FOR OBSERVING MINUTE INTERVALS OF TIME. 



The simple apparatus used by Professor Rood in measuring 

 the duration of a flash of lightning is described by him in a 

 recent communication, and he expresses the hope that others 

 may be induced to make use of something similar, in order 

 to add to our knowledge of this subject. Rood finds it difti- 

 cult to observe figures painted on a revolving opaque disk, 

 and considers it advisable to use a black or gray opaque disk 

 about four inches in diameter, having an open sector or square 

 hole cut through it. For flashes of medium duration he uses 

 a square of from one quarter to one half inch, and even larger. 

 For examining the multiple character of the flashes, he prefers 

 a long, narrow sector of one or two degrees. For portability, 

 a spring rotation apparatus is used, being so constructed as 

 to admit of giving the disk from twenty to thirty revolutions 

 per second, according to the observer's desire, and an index 

 should show to Avhat extent the clock-work is run down at 

 the moment of the observation of the flash, so that the rate 

 of rotation of the disk can be determined. Besides this port- 

 able instrument, another, a fixed one, is described by him as 

 calculated to give admirable results. 4 2>, Marcli^ 1873. 



NEW DETERMINATION OF THE VELOCITY OF LIGHT. 



A new experimental determination of the velocity of light 

 has just been eflected by Mr. Cornu, of Paris. The method 

 was that of Fizeau,in which a ray of light is sent to a tele- 

 scope at a distance of several miles, where it is reflected from 

 a mirror so that it returns directly to the observer. The ob- 

 server is supplied with a telescope, alongside of which a 

 toothed wheel revolves in such a way that the teeth pass di- 

 rectly across the focus of the telescope. The observer can 



