﻿certain Gases in Geisslers Tubes. 421 



illuminated with a brightness which only slightly diminishes 

 towards the more refrangible side. 



A very feebly illuminated field appears then at 66° 54' ; it is 

 of small breadth. 



12. If the condenser be placed upon Holtz's machine, the conti- 

 nuous spectrum changes at one stroke into the line- spectrum : 

 groups of lines stand out in places which were previously dark ; 

 the bright fields split up; and on the field (2), for instance, bright 

 lines start out right and left of the brightest part, while the 

 brightest part itself becomes dark. 



The colour of the light becomes bluish green. 



The positions of the individual groups of lines are, from several 

 concordant measurements, the following : — 



O I II 



1 st group of lines, left boundary . . . . 63 11 20 

 „ right boundary .... G3 19 30 



The right boundary is formed by a very bright line about 3' 

 distant from the preceding. 



2nd group of lines, left boundary . . . . 63 47 30 



In the middle a very large bright double line. 63 58 00 



Right boundary ... 64 8 40 



This group is formed from the green field which was before 

 designated as (2) ; it gives the impression that the brightest part 

 has been torn asunder at the left limit and separated into indi- 

 vidual lines. 



3rd group, of six lines . . between 64° 37' and 64° 46' 



The right boundary is the brightest. 



4th group of lines. It starts from the field 

 designated in the preceding section as (3), 

 yet in such a manner that the brightness of 

 the lines increases towards the more refran- 

 gible side ; the brightness has thus a distri- 

 bution the reverse of what it has in the con- 

 tinuously illuminated field. The left limit of 

 this group cannot, therefore, be quite sharply 

 determined ; several measurements furnished 

 values between 64° 58' and 65° ; so that the 

 beginning of this group does not coincide 

 with that of the bright field. The right t n 

 boundary is at 65 4 40 



5th group of lines, in the blue, three bright 



lines from 65° 40' 10" to 65 44 00 



6th. The bright field which, without the condenser, begins at 

 66° 18' 40", disappears entirely when it is added. Instead 

 Phil. Mag. S. 4. Vol. 37. No. 251. June 1869, 2 F 



