The Astrophysical Observatory 439 



sends a horizontal beam from north to south along the meri- 

 dian at a height of 1 10 centimeters from the floor, through 

 the tube T. 



The principal piece of apparatus, the spectro-bolometer, is 

 shown at D. This instrument, made by W. Grunow & Son, 

 is a development of that already devised by the writer and 

 figured by him in the "American Journal of Science."^ Its 

 object is to enable researches to be made on that invisible 

 portion of the solar radiation below the red in which it is now 

 known that a greater part of all the solar energy lies, in a 

 region whose details have been, up to the present researches, 

 comparatively unknown. 



This instrument consists of an azimuth circle of 52 centi- 

 meters diameter, reading by verniers to five seconds of arc. 

 Over the center of the azimuth circle is a prism, ordinarily of 

 rock-salt, a material pervious to the rays in question, which 

 do not freely pass through glass. This prism is fixed to a 

 mirror parallel to its rear surface, and it turns with it when 

 the circle is turned. A horizontal ray from the siderostat, 

 which falls upon the prism, passes through it at an angle of 

 minimum deviation, falls upon the plane mirror, and is by 

 that reflected to a distant concave mirror, 7n, by which an 

 image of the spectrum is formed at S. In the actual case, 

 the visible part of the solar spectrum is about nine inches in 

 length and one high, and filled with Fraunhofer lines, which 

 are visible to the naked eye when projected upon a screen. 

 The rays fall upon the strip of the bolometer at E. 



If, now, the circle be moved by the clockwork K, and with 

 it the prism and its attached plane mirror, the spectrum is 

 put in motion relatively to the bolometer strip, so that this 

 is virtually carried through the spectrum, its exact position 

 in it being at all times determined by reference to the circle. 



1 "The Selective Absorption of Solar Energy," Volume xxv, page 169, March 1S83, plate 2. 



