376 Mr. J. S. Ames on the Concave Grating 



of C, and is held there, free from all constraint, by a soft 

 wax. By means of the side and back screws the grating can, 

 then, be turned around its centre in its own plane, or tipped 

 back and forward. 



The slit placed at A is of somewhat complicated mechanism. 

 See fig. XVII. It has the following adjustments : — 



1st. Width of slit can be regulated by a micrometer-screw. 

 It is generally not open more than 0*001 in. 



2nd. The slit can be rotated about a central axis so as to 

 make it parallel to the lines of the grating. This adjustment 

 is one of the last to be made in mounting the grating, and is 

 done by turning the slit until the definition is the best possible. 

 This is most important, as the excellence of the photographs 

 depends largely upon it. The definition is spoilt, if the slit 

 is 0°-5 out. 



3rd. Stops can be inserted at top and bottom, thus causing 

 the grating to be illuminated by the centre of the solar image 

 only. Otherwise the definition may be spoilt by the rotation 

 of the sun. It is important, therefore, that the image of the 

 sun on the slit be quite large. With the larger apparatus in 

 use in the Johns Hopkins University it is 1*2 cm. in diameter, 

 and this is reduced one half by the stops. 



For solar work a heliostat, having a south exposure, throws 

 the light on the slit by means of a condensing-lens and a 

 totally reflecting prism. The lens is held in a brass frame, 

 and can be adjusted from within the building. Between the 

 prism and the lens is a revolving stage with circular openings, 

 across which absorbing solutions cin be placed. Both this 

 stage and the lever arm carrying the reflecting prism are 

 controlled by strings running along AC ; so that, without 

 leaving his seat, the observer can place different solutions 

 before the slit, or put aside the prism, when a metallic spec- 

 trum is to be photographed. For this purpose, along the 

 line of the slit and grating is a wooden tube with condensing- 

 lens, which focuses on the slit the image of the arc-light or 

 spark, placed in a separate compartment. (See fig. XII.) All 

 lenses and prisms must, of course, be made of quartz. 



For the arc-light a Weston dynamo of 150 volts, 30 

 amperes power is used, or alternating Siemens of 700 volts 

 maximum. For spark spectra, Professor Rowland has had 

 constructed an induction-coil, which (with from 3 to 12 

 gallon-jars) gives a spark of intense brilliancy, when driven 

 by the alternating Siemens dynamo. Using this coil, iron 

 wire T;Vi^i' diameter melts, and ^ in. wire is heated red hot. 



Gratings with 10,000, 14,438, and 20,000 lines to the inch 

 are used. For ordinary purposes a 10,000 one is sufiicient. 



1 



