326 Prof. J. A. Fleming and Mr. J. E.. Petavel : 



portion of the arc. In the diagrams figs. 13 to 17 are given 

 curves representing the periodic variation of the light from 

 the crater in the various cases, and extending over several 

 periods. It will be noticed that those diagrams represent- 

 ing the periodic variation of the light from the arc show- 

 that this light undergoes a regular fluctuation between a 

 maximum and a minimum, the maxima having equal values. 

 The light in the centre of the true arc never falls quite to 

 zero. This seems to be due to a little luminosity which 

 hangs in the interspace between the carbons, but at the 

 present moment it is difficult to say whether this persistence 

 is due to a very small admixture of stray light (although 

 every effort was made to keep this out), or to a persistence 

 of the illuminating-power of the incandescent vapour in which 

 the arc has been formed. On examining the true arc during 

 its complete periodic variation, it is found that the blue 

 or purple strip of light forming the true arc undergoes a 

 periodic variation in intensity. As far as the eye can judge, 

 the blue or purple light completely vanishes at a certain 

 instant during the phase ; but there is, outside the true arc, a 

 dim halo of golden light which is persistent; and it is therefore 

 probably on account of this persistent aureole of faint light 

 round the true arc that the ordinates of the carve representing 

 the periodic variation of the luminous intensity of the arc 

 never become zero, but always indicate the outstanding 

 constant amount of light. On the other hand, in the diagrams 

 which represent the periodic variation of light coming from 

 the centre of the crater of the lower carbon, we find the 

 luminous intensity of the crater varies between a minimum 

 value and two maximum values of different magnitude. 

 During the time when the crater is positive it reaches a higher 

 maximum intensity of illuminating power than during the time 

 when it is negative, and, moreover, the curve representing 

 the periodic variation of light rises more steeply than it 

 comes down, which indicates a slow cooling of the carbons 

 after they have been heated ; in other words, they heat more 

 quickly than they cool. This is particularly noticeable in that 

 part of curve corresponding to the crater being negative; and it 

 is only what would be expected, because after the carbon has 

 reached its negative maximum and is beginning to cool, the 

 opposite carbon is cooling from a condition in which it has 

 been positive, and as it has been heated to a higher tempera- 

 ture than the negative carbon, it must assist by its radiation 

 in keeping up the temperature and retard the cooling of the 

 negative carbon, These diagrams will also show many other 



