168 BARUS— ADJUSTMENT FOR PLANE GRATING [April 24, 



laboratory clamp K and a similar one on the upright c (screwed into 

 the carriage S) secures a small rod k for this purpose. Again a hole 

 may be drilled through the standards at K and c and provided with 

 set screws to fix a horizontal rod k or check. The rod k should be 

 long enough to similarly fix the standard on the slide 6^ carrying the 

 slit and be prolonged further toward the rear to carry the flame or 

 Geissler tube apparatus. The table tt is revoluble on a brass rod 

 fitting within the gas pipe, which has been slotted across so that the 

 conical nut M may hold it firmly. The axis passes through the 

 middle of the grating, which is fastened centrally to the table tt with 

 the usual tripod adjustment. 



2. Single Focusing Lens in Front of Grating. — I shall describe 

 three methods in succession, beginning with the first. Here a large 

 lens L, of about 56 cm. focal distance and about 10 cm. in diameter, 

 is placed just in front of the grating, properly screened and throw- 

 ing an image of the slit 6" upon the cross-hairs of the eye-piece E, 

 the line of sight of which is always parallel to the rod ab, the end 

 b swivelled in the carriage C, as stated (see Fig. 2). An ordinary 

 lens of 5 to 10 cm. focal distance, with an appropriate diaphragm, 

 is adequate and in many ways preferable to stronger eye-pieces. 

 The slit S, carried on its own slide and capable of being clamped to 

 c when necessary, as stated, is additionally provided with a long 

 rod hh lying underneath the carriage, so that the slit 5" may be 

 put accurately in focus by the observer at C. F is a carriage 

 for the mirror or the flame or other source of light whose spectrum 

 is to be examined; or the source may be adjustable on the rear of 

 the rod by which D and S are locked together. 



Finally the slide AB is provided with a scale .s.y and the position 

 of the carriage C read off by aid of the vernier v. A good wooden 

 scale graduated in centimeters happened to be available, the vernier 

 reading to within one millimeter. For more accurate work a brass 

 scale in millimeters with an appropriate vernier should of course 

 be used. 



Eye-piece E, slit S, flame F, etc., may be raised and lowered by 

 the split tube devise shown as at M and M' in Fig. 2. 



3. Adjustments. — The first general test which places slit, grating 

 and its spectra and the two positions of the eye-piece in one plane, 



