THE SPECTROGRAPH. 



171 



the graduated drum D. In other words, all inconveniences attendant either 

 upon the calibration of the spectroscope, or upon subsequent references to a 

 calibration curve, were avoided by the construction of the apparatus itself. 

 Secondly, all horizontal plane sections of the single glass prism were equal 

 trapeziums; consequently, the beam of light after entering the prism at the 

 face nearest to the collimator experienced one total interior reflection before 

 it suffered final refraction out into the air opposite the telescope objective. 

 This reflection diminished greatly the curvature in the field of view of all 



-G 



B 



FIG. 65 



spectral lines of which the light had not passed 

 through the prism at the angle of minimum 

 deviation. Therefore, it was possible to use 

 a greater length of slit and wider field of view 

 than would have been justifiable with the 

 usual type of triangular prism of equal disper- 

 sion. The chief practical advantage of the wide field of view was that two 

 different spectra could be seen in this field, the one above the other, and 

 readings could be taken directly from the graduated drum D, without appre- 

 ciable danger arising from curvature of spectral images. 



THE CELLS. 



For photographic purposes two cells were used in the study of aqueous 

 solutions. One of these is shown in vertical section in fig. 66. The dia- 

 gram is of natural size. The cell was designed to fulfil five conditions: 

 (a) To transmit without sensible selective absorption all radiations between 

 wave-lengths 0.20// and O.SO//; (6) not to be acted upon chemically by the 

 solutions placed in the cell; (c) to cause the incident and emergent surfaces 

 of the absorbing liquid to be plane and practically parallel; (d) to be capable 

 of adjustment with respect to the length of column of liquid traversed by 

 the light; (e) to keep itself as clean as possible when in the immediate vicinity 

 of the spark. The parts of the cell may be briefly described as follows: 

 D and F were short sections of glass tubing, the ends of which were ground 



