Absorption Spectra of Dilute Solutions. 



121 



internal diameter, and the longest (length = 1 metre) was provided 

 with five rings of ebonite (2 3 mm. wide), distributed at equal dis- 

 tances along its inside in order to prevent reflexion from the walls. 

 The shorter tubes were provided with black paper rings at their ends 

 for this purpose. After passing through the solution, the light is 

 reflected internally by a right-angled flint-glass prism E, and falls on 

 the Lummer and Brodhun prism F.* The construction of this 

 photometric prism has been described minutely by Lummer and 

 Brodhun (' Zeitschrift fiir Instrumentenknnde,' 1889, p. 461 ; and 

 1892, p. 46). It consists of two right-angled prisms, ground and 

 polished together on their hypothenuse surfaces, and held together as 

 shown in fig. 2. Certain parts of the surface of one prism are removed, 

 at a, a for example, so that the light coming from E is totally reflected 

 at the surfaces a, but transmitted at b. The light from lens b is only 

 transmitted at the surfaces b. Consequently, an eye placed at O sees 

 the surface 6, 6, 6, a, a, illuminated on the parts marked a by light 

 from E, and on the parts b by light from the lens b. The measure- 

 ment then consists in varying the brightness of the light coming 

 from lens 6, until the brightness of the different surfaces is the same. 

 By means of glass plates at d, d (fig. 2), a reading by equal contrast 



FIG. 2. 



between two pairs of surfaces of somewhat different brightness, can 

 also be made at the same time. For the details of this arrangement, 

 however, I must refer to Lummer and Brodhun's original papers. 



The variation of the intensity of the light from lens b was pro- 

 duced by means of the rotating sector described by Abney (* Phil. 

 Mag.,' vol. 27, p. 62, 1889). The sector was made by Mr. A. Hilger. 



After passing through the prism F, the two beams of light follow 

 the same path through the two colourless flint-glass prisms G and I (one 

 of 60, the other 45). The lens g then throws the spectrum thus pro- 



It was made by Messrs. Schmidt and Haensch, of Berlin. 



K 2 



