43 6 The Smithsonian Institution 



which alone can correspond with any given inflection of the 

 curve. 



The theory is simple, but the practice is extremely difficult, 

 and it has, in fact, consumed nearly five years of continuous 

 labor to produce the results which are obtained by the pres- 

 ent apparatus, which works in the following manner : 



A beam from the mirror of the siderostat is conveyed 

 through the slit of a telescope having a rock-salt objective of 

 about ten meters focal length to the prism, which is mounted 

 on the massive spectro-bolometer, the novel feature lying in 

 the mechanical connection of the large circle carrying the 

 prism with a distant photographic plate, susceptible of verti- 

 cal motion, and taking the place of the scale formerly in front 

 of the remote galvanometer, both circle and plate being now 

 moved by the same clockwork, through a continuous train of 

 shafting, which works with such steadiness and precision as 

 to make the two movements entirely synchronous. 



To understand this better, let us suppose that the very 

 slowly moving circle carrying the prism moves the spectrum 

 through one minute of arc in one minute of time, across 

 the vertical bolometer thread. To the observer watching the 

 spectrum the motion is as slow as that of the hour-hand on the 

 dial, but it is continuous and uniform, and the same mechan- 

 ism which causes this motion of the spectrum of one minute 

 of arc in one minute of time causes the photographic plate to 

 move vertically, before the galvanometer mirror, at any given 

 rate, for instance, at the rate of one centimeter of space in 

 one minute of time. It follows that during every second of 

 this minute a portion of the spectrum represented by one 

 second of arc will have glided before the bolometer thread, 

 and that during this same second the photographic plate will 

 have been lifted automatically through one sixtieth of a centi- 

 meter in space ; the essential thing being that the plate shall 



