﻿E. C. Pickering — Ultra Violet Spectrum. 223 



mercury reduced the galvanometer deflection from 36 to 35 

 divisions of the scale. This indicates a change of less than 

 three per cent, resulting from a pressure which with the soft 

 disk lowered the resistance by more than sixty per cent. There 

 can be little doubt that this small reduction is due almost 

 entirely to better surface contact produced by pressure. 



Throughout all of the experiments with soft carbon, it ex- 

 hibited more or less irregularity in its behavior. The applica- 

 tion of a pressure very largely in excess of the maximum 

 referred to above would sometimes result in a permanent 

 reduction of the resistance of the disk, indicating that a per- 

 manent set had taken place. By the exercise of care, however, 

 what may be called the " normal " resistance may be main- 

 tained fairly constant for a considerable length of time. 



Conclusions. — When carbon is prepared in the form of com- 

 pressed lamp-black, its electrical resistance varies greatly with 

 the pressure to which it is subjected. A small part of this 

 variation is doubtless to be attributed to change in surface con- 

 tact between the carbon and the electrodes through which the 

 current is introduced, but by far the larger part (provided any 

 effort is made to secure good surface contact) is due to a real 

 change in the resistance of the carbon itself. The resistance of 

 carbon in this condition is fluctuating and uncertain to a degree 

 that seems to prevent its use as a factor in any device for the 

 accurate measure of pressure. 



Art. XX VI. — Comparison of Maps of the Ultra Violet Spectrum ; 

 by Edward C. Pickering. 



One of the first accurate maps of the ultra-violet portion of 

 the solar spectrum was made in 1873 by the late Dr. Henry 

 Draper. A photograph of a normal diffraction spectrum was 

 prepared and copies printed by the Albertvpe process. A 

 wide distribution was given to this work, which was published 

 also in this Journal, cvi, p. 401. This map extended from 315 

 to 391, adopting as a unit the millionth of a millimeter. The 

 scale was such that one unit equaled 0'31 cm . Soon after, M. 

 Cornu published in the Annales Scientifiques de l'Ecole Kor- 

 male Superieure, III, 121, a steel engraving representing the 

 region extending from 313 # 5 to 412'5. The map was intended 

 as a continuation of the work of Angstrom and was on the 

 same scale, one unit equaling l cm . 



The recent publication of a photograph of the solar spectrum 

 by Professor Eowland furnishes a convenient standard with 



