THE SPECTRUM 305 is equally focussed 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 he obtained with the oxy- hydrogen light. The arc-light requires modification. The positive carbon (crater) must be the loiver 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