of the Interrupted Spectra of Gases. 43 



Foucault's determination of the velocity of light, viz. 298,000,000 

 metres per second, the value of this constant is 



t= 3-3557 twelfth-seconds, .... (2) 



meaning by a twelfth- second a second of time divided by 10 12 , 

 which in other words is the millionth part of the millionth of a 

 second of time. 



Section II. On the periodic time of one of the motions in the 

 molecules of Chlorochromic Anhydride. 



5. We generally made use of the vapour of chlorochromic an- 

 hydride mixed with air, and at the temperature and pressure of 

 the atmosphere. We tried the vapour freed from air, and also 

 at somewhat higher temperatures ; but in neither case did we 

 observe any marked difference in the spectrum. An image of 

 the lime-light was formed on the slit of the spectroscope by a 

 condensing-lens of about 30 centims. focus, and of such a size 

 that the whole of the collimating-lens was filled with light*. 

 The column of chlorochromic vapour, if not too longfj was placed 

 between the condensing-lens and the slit. The spectrum which 

 then presents itself consists of a number of sensibly equi- 

 distant dark lines in the orange, yellow, and green. In the 

 orange these lines fade away and leave part of the orange 

 and the red unsubdued by lines. In the other direction the 

 lines are gradually lost in an increasing obscuration of the spec- 

 trum, which entirely blots out the more refrangible colours. The 

 lines are nowhere sharply defined or narrow, nor are the spaces 

 between them devoid of duskiness % \ and the region of general 

 absorption, which occupies the more refrangible part of the spec- 



* This is a condition which is essential to making good measures. If 

 only a vertical strip of the collimating-lens is supplied with light, the beam 

 belonging to one line in the spectrum reaches the image in the observing- 

 telescope as a thin wedge of light, upon which the eye does not readily 

 focus itself. Under these circumstances the eye keeps continually altering 

 its adjustment, feeling about for the right distance; and if the strip of 

 light has not passed exactly centrally through the instrument, this causes 

 the image of the line to appear to deviate in various degrees from its true 

 position, accordingly as the adjustment of the eye fluctuates. 



t We used columns of various lengths, varying from 4 to 80 centims. 

 From 15 to 20 centims. is a good average length. By increasing the length, 

 the lines in the yellow and orange become more conspicuous ; and by dimi- 

 nishing it the lines in the green are better seen. With the longest column 

 we were obliged to place two condensing-lenses at the ends of the tube, one 

 to send the light from the lime-light in a parallel beam along the tube, and 

 the other to condense this parallel beam into an image upon the slit of the 

 spectroscope. 



% See further on, § 22. 



