THE SPECTRUM 305 



is equally focused upon the screen, from end to end. To all 

 * line ' work of any kind this is particularly essential ; and it 

 is never the case unless the lens is specially adjusted for it. 

 The simplest method is to hold a fine wire across the slit ; 

 and then to adjust the lens for focus, and incline the lens, 

 until the shadow of the wire appears as a black line equally 

 sharp from one end to the other of the spectrum. 



176. The Light. A jet of the most powerful kind should 

 be used, with plenty of pressure and the very best limes. 

 Only thus can respectable results be obtained with the oxy- 

 hydrogen light. 



The arc -light requires modification. The positive carbon 

 (crater) must be the lower one for spectrum work ; and both 

 carbons should be perpendicular and in line. The Brockie 

 lamp can be used without difficulty, by inserting a wedge- 

 shaped block under it so as to bring the carbons upright ; and 

 the current may be reversed by providing an extra special 

 carbon-holder for the lower pole, which must protrude con- 

 siderably. The lower carbon cannot then feed up to the 

 usual stop, and therefore the focus will have to be adjusted 

 from time to time, by the screw motion of the table. Spec- 

 trum experiments do not, however, require any very great 

 accuracy in focus-keeping ; and most demonstrators prefer 

 a simple hand rack-regulator, moving the two poles in due 

 proportion, and which, with a screw table as described in 

 Chapter XII. will answer all purposes, at a small cost. Such 

 hand-regulators are certainly best for the large excavated 

 carbons presently mentioned, and allow of the length of arc 

 being varied with much greater facility. 



177. Simple Absorption. For solutions, thick glass cells, 

 including some made of a wedge shape, are very convenient, 

 as showing the effect of stronger and stronger absorptions in 

 coloured media. With the two prisms and the spectrum 

 nicely focussed, the definite and local character of colour- 

 absorption may be contrasted with the general absorption 



X 



