( . s ( ook — Spectrum qfAqusous Vapor. 



269 



a" is 



the angle which the tube makes with the horizon. At i% 



the Bcrew by which the telescope is turned to view different 



parts of the spectrum. The attachment which permits of a 



photometric measurement of absorption lines appears at "»•." 



The object sought is the production of an artificial line 

 whose intensity can be varied at will. This fictitious line is 

 placed alongside the spectral line which is to be measured, and 

 the intensity of the former is then varied until the two appear 

 like. As this variation in the blackness of the artificial line is 

 effected by the use of a micrometer screw, its readings consti- 

 tute an arbitrary value of intensities. The artificial lines 

 (there are a pair of them) are diffraction fringes. They are 

 produced by a single silk fiber placed slightly beyond the 

 focus of the eye-piece. As seen in a telescope or microscope, 

 such fringes are quite indistinct ; but in the spectroscope, the 

 fiber is in light which is practically monochromatic, which 

 greatly favors distinctness of definition. Much improvement 

 is effected also by viewing the spectrum through a narrow 

 opening parallel to the fiber and consequently parallel also to 

 the spectral lines. A piece of blackened brass is placed on the 

 eye lens next the eye. In this cover is cut a slit one milli- 

 meter wide, extending entirely across the lens. It is found that 

 the definition of the artificial line is improved if this observing 

 slit is eccentric. The edge of the slit should be placed at the 

 center of the eye lens, making the slit one-half a millimeter 

 displaced upward or downward. 



