12 ATLAS OF ABSORPTION SPECTRA. 



even in a tightly stoppered bottle, and is best made up fresh just before 

 being applied as an adhesive. 



Since the completion of the experimental work on the aniline dj-es a 

 cell, in the construction of which no cement at all was employed, has been 

 designed and successfully used by one of us.* This cell could retain any 

 liquids which would not attack glass and quartz and, although it was 

 designed to confine the solutions in plane-parallel layers, nevertheless, the 

 principles involved in its construction were such as to admit of extension to 

 the production of a cell which would be wedge-shaped in the interior and 

 would, at the same time, hold organic solvents, prevent evaporation, etc. 



THE SPECTROGRAPH. 



The essential parts of a vertical section of the spectrograph are outlined 

 in figure 6. They may be tersely described, with the aid of symbols, 

 as follows: In the first place, the elements of the system were adjustable 

 in every respect. Light from the Nernst filament, N, was focused by the 

 concave speculum mirror, R, on the slit, S, whence it continued to the 

 grating, G, from which a portion of it was dispersed in the direction of the 

 sensitized film, F. The distances from the middle of the slit to the centers 

 of the mirror and grating were respectively about 89. 5 cm. and 97. i cm. 

 The electrodes, E, were usually at the distance of 4. 2 cm. above the slit and 

 they did not interfere with the passage of the light from the reflector to the 

 slit. No lenses or other reflectors were used. The micrometer head at M 

 indicated the separation of the slit-jaws. Q and Q' denote a screen system 

 such that when Q was vertical the passage of light from the grating to the 

 camera was not interfered with, whereas when Q was horizontal only ultra- 

 violet light of shorter wave-length than 0.4,^ could reach the photographic 

 film. PP is a horizontal platform with a scale along its front edge. By 

 sliding projecting, horizontal, opaque screens of various widths along this 

 platform it was possible to cut out completely any region or regions of wave- 

 lengths desired. 



In making certain tests, the platform and sliding screens were very 

 convenient. L is the section of a thin, black, metal shutter capable of 

 motion in a horizontal direction and hence at right angles to the length of 

 the photographic films ; in other words, parallel to the slit and to the rulings 

 of the grating. A number of long, rectangular slots or openings, suitably 

 spaced and proportioned, were present in this screen so that strips of differ- 

 ent widths of the films or plates could be exposed to the light from the 

 grating without causing any displacement of the sensitized surfaces with refer- 



*The description of the details of the cell is given on pages 241 to 243 of Publication No. 60 of the 

 Carnegie Institution of Washington, entitled : Hydrates in Aqueous Solution. By Harry C. Jones. 



