424 Prof. D. B. Brace on a New Spectrophotometer 



that the cross-hairs bisect the sodium line. The two spectra 

 are then found to be exactly superimposed throughout their 

 extent. On removing the eyepiece and inserting in its stead 



Fig. 3. 



an ocular slit usually about 2 millim. high and "5 to 1 millim. 

 wide, and observing the prism directly with the eye, a circular 

 field crossed by a horizontal band is seen, which latter vanishes, 

 giving a uniform tint over the entire aperture, when the 

 intensities of the two sources are properly adjusted by varying 

 the slits. When the telescope R is shifted continuously 

 through an angle of about 6°, the field appears lighted up 

 uniformly with all the spectral colours in succession. The 

 slightest fluctuation in the light or disturbance of the instru- 

 ment manifests itself at once in the outlining of the horizontal 

 strip. The advantage of a single prism in obtaining the 

 same effect as with a prism and the cube of Lummer-Brodhun 

 is evident in the simplicity of adjustment and the elimination 

 of optical defects arising from a greater number of reflecting 

 and refracting surfaces, only one prismatic refraction occurring 

 after the rays leave the silvered strip. The symmetry of the 

 optical system allows the use of the ordinary spectroscope 

 and spectrometer as a spectrophotometer by the addition of 

 an extra collimator. 



Method of Calibration. 



One of the serious difficulties in spectrophotometry is in 

 determining the relative intensities of the sources. The 

 direct use of the method of varying the distance is too cum- 

 bersome for colour-work, while the composition of the light 

 may be changed by admixture from extraneous sources. 

 The direct use of Nicol and other polarizing prisms during 



