296 BEPOET— 1880. 



under different siDark-conditions would be of great value. The relative 

 intensity of the two strongest hydrogen lines in a series of experiments 

 was estimated by the Greenwich observers.' Messrs. Trankland and 

 Lockyer ^ had already pointed out that on a reduction of pressure the 

 blue line (F) is the last to disappear ; and Mr. Lockyer^ afterwards pointed 

 out that an increase of temperature made the red line stronger, In the 

 published results of the Greenwich observations the pressure of a va- 

 cuum tube varied between 387 mm. and 100 mm. The relative intensities 

 are given as follows : — 



Pressure = 387 mm. Ha = 10 Hj8 = 10 



297 10 8 



217 10 8 



100 10 9 



In the last case the F line was considerably narrower than before, and 

 this may have caused the increased brilliancy. 



Through the kindness of Mr. Christie, I am also enabled to give some 

 unpublished observations on the same point made at Greenwich. A tube 

 sealed off under a pressure of 5 mm. was employed and the effect of an 

 air-break in the circuit was studied. Without an air-break Ha was brighter 

 than H/3 by one-fourth ; a break of one inch introduced little change, 

 except that the whole spectrum was fainter ; but when the break was 

 increased to 1"75 inches, H/3 was the brightest line. Ha being only about 

 two-thirds as bright. In these experiments the spark was sufficiently 

 strong to cross an air space of 2 '5 inches when the tube was not inter- 

 posed. Other experiments with a somewhat stronger spai'k confirmed 

 the fact thatf H/3 is increased in relative intensity when the break is 

 increased. 



With a spark which could cross a space of I'S in. and a break of 

 1'75 in. the following observation was made: — 



The spai-k passed occasionally with great difficulty, sometimes giving 

 the usual crackle and at other times a sharp report, almost like a pistol. 

 The degree of ease with which the spark passed, and the appearance of the 

 spectrum, both varied so rapidly that it was difiicult to say if one depended 

 on the other. So far as could be ascertained. Ha was absent when the spark 

 passed with the greatest difficulty, and brightest when it passed most easily.' 



M. Lecoq de Boisbaudran ■* mentions that an increase of temperature 

 is often accompanied by a relatively greater increase in the brilliancy of 

 the more refrangible rays. It is often said that such an increase is a 

 direct consequence of the formula established by Prof. Kirchhoff. If 

 the absorptive power of a molecule remains the same, while the tempera- 

 ture is increased, it follows that the blue rays gain more quickly in inten- 

 sity than the red ones, but the less refrangible ones would never actually 

 decrease in intensity, the quantity of matter remaining the same. Now 

 such a decrease is observed in most cases mentioned by Lecoq de Bois- 

 baudran, and there is generally no reason to suppose that the quantity of 

 luminous matter has been reduced. We may doubt, therefore, that the 

 observed differences in the spectra are in all cases regulated only by 

 Kirchhoff's law ; but it is a perfectly plausible hypothesis that a higher 

 temperature is in general accompanied by a decrease in the absorptive 

 power of the less refrangible rays. As a stronger blow often brings out 



> Greemriclt Astron. Hemlts, p. 121 (1875). ^ Proc. Boy. Soc. xxiv. p. 352 (1876). 

 ^ Proc. Roy. Soc. xviii. p. 79 (1869). •• Spectres Lumincux, texte p. 43 (1874). 



