The Astrophysical journal . manner, while the other electrodeis the brass casting itself. Thus, when the apparatus is exhaustedthe discharge pisses from the platinum along the capillary to thesides of the casing, the end of the tube, some mm in diameter,furnishing a bright point of light. The bits of the substances whosetransparencies are*to be examined are fastened over the holes of thedisk shown in the illustration. This disk can be rotated through asimple cog mechanism, by a key fitting air-tight in the screw coverwhich closes the end of the casing tube. By this rotation the speci-mens
The Astrophysical journal . manner, while the other electrodeis the brass casting itself. Thus, when the apparatus is exhaustedthe discharge pisses from the platinum along the capillary to thesides of the casing, the end of the tube, some mm in diameter,furnishing a bright point of light. The bits of the substances whosetransparencies are*to be examined are fastened over the holes of thedisk shown in the illustration. This disk can be rotated through asimple cog mechanism, by a key fitting air-tight in the screw coverwhich closes the end of the casing tube. By this rotation the speci-mens are brought in turn before the end of the capillary and at adistance of about 6 mm from it. The mirror employed is of speculum, radius of curvature 25 cm,and the mounting is of a very simple form permitting movements 1 Loc. cit., p. 196. ABSORPTION FOR SHORT WAVE-LENGTHS 47 about horizontal and vertical axes. The prism is of excellent whitefluoritc, by Zeiss, angle 6o°; it is mounted on a table fitted withleveling Fig. i Fig. The exact dimensions are not essential to the success of the appa-ratus; Fig. i is one-third, and Fig. 2 is two-thirds, of natural is worth noting that the screw-joints which close the prism-chamberand one of the casing tubes have proved more satisfactory than 48 THEODORE LYMAN flanges with ground plates. In order to secure an air-tight joint, itis necessary only that the screw-threads be carefully cut and wellcovered with stopcock grease. Brass apparatus made from castingsare almost sure to contain pin-holes, but the resulting leak may bereduced to a minimum by the use of thin shellac. It is obvious that the optical arrangement of the apparatus issuch that no great perfection of definition is to be expected. Forthe rough tests for which the instrument was constructed this, how-ever, is not a serious drawback. The operation of the instrument is as follows. The mirror andprism are so adjusted that the lines at the extreme ultra-violet end
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectspectru, bookyear1895
