. Carnegie Institution of Washington publication. 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 telescop
. Carnegie Institution of Washington publication. 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 and ; (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
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