
Bright Spectrum Lines. AQ] 
corresponds to one turn of the screw. The other is a tangent 
screw by which it is possible to give the carriage such a 
slow motion that the change ae one fringe to the next 
ean be easily followed. To both handles were attached 
graduated disks enabling the distance between the plates to 
he accurately known 
The whole instrument weighed over 15 kilograms and was 
placed on a brick pier. The greater part of the observations 
were taken at night. With this instrument the fringes were 
always perfectly steady, and very long photographic exposures 
could be made without the least fear of obtaining a blurred 
image. 
Since the radiations from all the sources studied consisted 
of many wave-lengths it was necessary to employ some 
arrangement by which the wave-length under aa 
could be separated from the others. The following (fig. 2) 
was the plan first adopted. 8 is the source of light. The 
Fig 
2 
Se —s 

Ss 
radiation undergoes an analysis by a Steinheil spectroscope 
consisting of two flint-glass prisms. ‘The lens L brings the 
different wave-lengths to a focus on a screen I which contains 
a slit. Through this slit the wave-length considered is 
allowed to pass, and passing between the silvered plates forms 
ia? | 
the interference-bands, which are observed by a telescope T 
or photographed. 
The photographic apparatus consisted of a long light- 
proof box with a circular hole cut in one side. The eye- 
piece of the telescope being removed, the box was so placed 
that the opening fitted over the end of the telescope. The 
photographic plate, 13 by 3 cms., was in the focus of the 
objective and mounted so that it could be slid past the 
