232 
MESSRS. W. DE LA RUE AND H. W. MULLER ON THE 
It also changes the spectrum of the strata ; moreover, the spectra of the 
illuminated terminals and the strata difter. 
6. If the discharge is irregular and the strata indistinct, an alteration of the 
amount of current makes the strata distinct and steady. Most frequently a 
point of steadiness is produced by the careful introduction of external 
resistance ; subsequently the introduction of more resistance produces a new 
phase of unsteadiness, and still more resistance another phase of steady and 
distinct stratification. 
7. The greatest heat is in the vicinity of the strata. This can be best observed 
when the tube contains either only one stratum, or a small number separated 
by a broad interval. There is reason to believe that even in the dark 
discharge there may be strata ; for we have found a development of heat in 
the middle of a tube, in which there was no illumination except on the 
terminals. 
8. Even when the strata are to all appearance perfectly steady, a pulsation can 
he detected in the current ; hut it is not proved that the strata depend upon 
intermittence. 
9. There is no current from a battery through a tube divided by a glass division 
into two chambers, and the tube can only be illuminated by alternating 
charges. 
10. In the same tube and with the same gas, a very great variety of phenomena 
can be produced by varying the pressure and the current. The luminosities 
and strata, in their various forms, can be reproduced in the same tube, or in 
others having similar dimensions. 
11. At the same pressure and with the same current, the diameter of the tube affects 
the character and closeness of the stratification. 
Our special thanks are due to Professor Stokes for many valuable suggestions and 
criticisms during the course of our investigation. 
W e have very great pleasure in stating that during the last twelve months we have 
had the benefit of the zealous assistance of Mr. W. Sharpey Seaton, formerly in 
the service of Professors Sir Wm. Thomson and Fleeming Jenkin, and that we have 
found his familiarity with electrical measurements, and his resources in devising 
methods of overcoming the many impediments we have encountered in connexion 
with them, of the greatest value. Mr. Fram has continued to give us zealous, 
intelligent, and patient assistance, and has helped us over many difficulties. The 
photographs copied in Plates 15-18 were taken by Mr. H. Reynolds, Mr. De La 
Rue’s former assistant in astronomical photography.* 
Our experiments are still going on, and we may have, at a future time, some more 
* Phil. Trans. 1862, pp. 333-416. 
