S. P. Langley on the Selective Absorption of Solar Energy. 175 



the rajs from the prism, which has first been replaced upon 

 its table and adjusted by the screws below. The prism is now 

 carefully set to minimum deviation (usually for the D l line), 

 and is then automatically kept in minimum deviation for all 

 other rays by the tailpiece and attachment at D. When the 

 cross wires of the eyepiece are set upon the D line, the circle 

 should indicate a deviation of 47° 41/ 15". A bright and 

 pure image of the spectrum, about 6 millim. wide and 640 mil- 

 lim. long, between the A and H lines is now formed in the 

 principal focus of M near the prism ; and the bolometer-case 

 being substituted for the eyepiece, the carriage is slid along 

 the track until the central strip, placed vertically and parallel 

 to the Fraunhofer lines, comes exactly into focus. The heat 

 of the solar rays in any part of the spectrum may now be 

 measured by the bolometer (the galvanometer giving a marked 

 deflection as it passes over the leading Fraunhofer lines) ; and 

 the deviation for that part is exactly indicated by the divided 

 circle. 



The galvanometer used in connexion with the bolometer is 

 a Thomson reflecting astatic galvanometer of about 20 ohms 

 resistance, constructed especially for the purpose by Elliot 

 Brothers, of London. It is placed upon a pier entirely dis- 

 connected from the building. The scale is cylindrical, with 

 divisions 1 millim. apart on a transparent surface, and is 

 placed 1 metre from the galvanometer mirror. Since the 

 whole deflection ordinarily employed does not exceed 5°, as a 

 rule the reading of the galvanometer requires no reduction for 

 our present purpose. A resistance-box forming the Wheat- 

 stone's bridge and other electrical adjuncts of the bolometer 

 are on the right of the galvanometer-pier. The rheostat is in 

 a convenient position near the scale ; and the battery-galvano- 

 meter, for measuring and regulating the strength of the cur- 

 rent used, is on a pier in another part of the room. 



In conducting the measurements for mapping the spectrum, 

 one observer is usually at the spectro-bolometer to set the 

 circle to the deviation required, to see that the light from the 

 siderostat falls properly upon the prism, and to admit the sun- 

 light at a given signal by means of cords attached to a sliding 

 cover in front of the slit where the sun's rays first enter the 

 room. Another observer, placed at the galvanometer, reads 

 the corresponding indications of the instrument; and a third 

 enters them in form in a book prepared for the purpose, and 

 gives the signal for exposure. As all these observations are 

 carried on in a partially darkened room, a fourth person is 

 usually stationed without to wind the siderostat clock, and to 

 give notice of any passing clouds to those within the building. 



