THE ABSOHKnON AN1> EMISSION OK All!. 



Fig. [) is tlit; liorizoiitiil plan of the spectrogr.ipLi. The slit is shown at '/", with 

 the colliiiiator-tube, e, in which there is a peep-hole, w,, iiidispeusable for the 

 atljiistrnent of the lens. The telescope has a similar peep-hole, it 2- The telescope 

 oari'ies the camera, which is provided with handles, l', for the purpose of turning 

 its interior cone, b'. Upon the outer cone of the camera the drawing shows a 

 divided arc whoSe divisions are whole degrees. This serves for the adjustment of 



KiG. 8. Elevation of Spectrogrami. Half Natural Size. 



the inclination of the photographic plate to the optical axis of the lens; which de- 

 ])ends upon the refrangibility of the rays to which exposure is to be made. AN ith 

 a sixty-degree prism of fluor-spar, this angle of incidence is 08" when light of w-ave- 

 length 185 f^n falls on th(' middle of the plate. From that point on, it diminishes 

 considerably with the wave-length ; so that for the most refi-angible region photo- 

 graphed l)y me it only amounts to i)5°. The camera has a counterpoise, q. At »• 

 is the index for the setting of the angle between the optical axes of the two lenses 



