ELEMENTAKY PHYSIOLOGICAL CHEMISTKY 



197 



may be obtained by directing the instrument towards a white cloud, 

 and moving the eye-piece till the various Fraunhofer lines are clearly 

 defined, or, in absence of daylight, obtaining a clear image of the upper 

 and lower edges of the slit, i.e. of the upper and lower edges of 

 the spectrum. The slit should not be too widely open. If the source 

 of light include a sodium flame, a clear image of the D-line will be 

 obtained when the slit is in focus. 



FIG. 144 B. 



FIG. 144. Spectroscopes. A, Small direct vision spectroscope. 

 B, Compound spectroscope. 



In the larger forms of spectroscope, three tubes are generally found radiat- 

 ing from the central prism or prisms 1 (Fig. 144B). One of these has its end 

 blocked by a screen, in which there is a slit where width can be varied 

 by a small screw. Attached near to the slit there is generally a small 

 prism, which can be moved so as to cover half the slit, and affords the 

 means of introducing a second source of light into the instrument. The 

 tube, at the end of which is the slit, is called the collimator tube, and 

 contains a lens so that the image of the slit can be brought to bear 

 on one face of the prism. The distance of the slit from the lens 

 is variable, and should be adjusted so that the rays issue from tube into 

 the prism as parallel rays. After refraction through he prism they are 



1 For studying absorption spectra, the two-prism form with its greater 

 dispersion of the spectrum is less well adapted. 



