130 LYMAN. — TIIE SPECTKUM OF HYDROGEN". 
ADJUSTMENT. 
After the spectroscope is placed in the receiver the grating is turned until that 
part of the first spectrum to be investigated falls on the photographic plate. The 
arrangement of two slits serves a double purpose, as by it either the method of shifted 
spectra or the second spectrum comparison method may be used, without altering the 
position of the grating. For no matter which method is to be employed the grating 
is so placed that light from the right hand slit gives the region of short wave-length 
in the first spectrum, while by illuminating the left hand slit a shifted first spectrum is 
obtained superposed upon a shifted second spectrum. The dimensions of the apparatus 
are such that when the longest wave-length which falls on the plate from the right 
o 
hand slit lies in the region of 1900 Angstroms, the longest wave-length in the shifted 
first spectrum has a value of about 3100 Angstroms. Observation of lines in the 
shifted spectrum serves, therefore, as a simple test of the exact position of the grat- 
ing. When this position has once been reached the erratinsr end of the receiver is 
&*"■ "~o 
closed, a very little vaseline being used in the joint, and the edge is looted with shel- 
lac or De Kotenski cement of the softer kind. It next becomes necessary to prepare 
face plate. If the shifted spectrum method is to be employed 
pi 
consists in covering that hole which is to admit light to the left hand slit with a 
quartz window and to seal the discharge tube over the right hand opening. This last 
adjustment is a tedious one, for the mouth of the discharge tube must be ground at 
such an angle that the capillary lies in the line determined by the slit and the grating 
centre. This can only be done by trial. When the correct angle has been arrived at 
the tube is fastened to the face plate with De Kotenski cement. To insure a stroi 
joint the brass surface must be heated during the operation. The face plate with t] 
tube thus attached is rubbed evenly with a little white vaseline and applied to the 
flange of the receiver. Here great care must of course be used that the tube is in line 
To facilitate this operation, tubes of both forms are made double ended, 
o 
the 
they have a quartz window by means of which it is possibl 
capillary to the slit and 
look th 
6 
Once in position the pi 
clamped and the edge looted with cement. Plate VII, fig. 4, illustrates the appe 
~ of the more improved form of plate and discharge tube in position. 
The fact that the end of 
i 
dark closet permits the plate-holde 
fo? t! 1 ", , IT ° l the S P eCtrosc °P e th ™S» the hand-hole without danger of 
jo:; a Ltz,;r t* by th ; conicaI ph,g and around the edge of the 
spread. The apparatus is now ready to exhaust. If no window 
