170 HYDRATES IN AQUEOUS SOLUTION. 



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 shut- 

 ter capable of motion in a horizontal direction, and hence at right angles to 

 the length of the photographic films or plates; 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 different widths of the films or plates could be exposed to the 

 light from the grating, without causing any displacement of the sensitized 

 surfaces with reference to the grating and slit. This was necessary for im- 

 pressing comparison spectra, etc. H and H' suggest the rack and pinion 

 system, by the aid of which the films could have unexposed portions brought 

 successively opposite to some selected opening in the slide screen L. D and 

 D' denote two of the four doors that gave access to the interior of the 

 spectrograph, and which made it possible to close up the camera light-tight, 

 while making various adjustments with the rest of the system. The camera 

 was so made that when neither a film nor plate was in position, it was possible 

 for the experimenter to look directly at the grating and to make qualita- 

 tively, observations with the assistance of any eyepiece. Certain black-on- 

 white scales and ruby-glass windows (Z, for example) enabled the experi- 

 menter to know the precise relative positions of the various accessories in 

 the interior of the spectrograph when the entire system was shut up and 

 exposures were being made. Numerous dull black diaphragms and screens 

 (Ai, A 2 , AS, A, A 5 , etc.) protected the photographic film from the unus- 

 able light which came from the central image /, and from all the spectra 

 except the one desired. U\ and 0\ gave the extreme rays of as much of 

 the first-order spectrum as was studied photographically; that is, U\ and 

 0\ correspond, respectively, to about 0.20,". and 0.63/;. The spectrograph 

 was of course dull black both inside and out, and contained plaited black 

 velvet wherever needed. A general idea of the size of this apparatus may 

 be derived from the following dimensions: From R to the plane of BC = 

 198.5 cm.; Z?C = 34.5 cm.; the bottom edge perpendicular to BC = 27.5 cm.; 

 BJ=11G cm., and JK = 29 cm. 



THE SPECTROSCOPE. 



This instrument was of the direct-reading type, and was obtained from 

 Hilger, of London. An orthagonal projection of it on a horizontal plane is 

 shown at H in fig. G5. Two details deserve special mention. In the first place 

 the instrument was so designed that when the prism was properly adjusted 

 on its turn-table, and the cross-hairs were correctly placed in the telescope, 

 the wave-length of any visible spectral line which appeared to be bisected by 

 the point of intersection of the cross-hairs could be read off directly from 



