The Astrophysical Observatory 435 



greater rapidity, but with greater certainty, and by an auto- 

 matic process. The idea in its original simplicity is very 

 easily understood. 



In the old process, just described, the deflection of a spot 

 of light upon a scale was read by one observer, while another 

 simultaneously read the position in the spectrum of the cold 

 band, or line, which caused the thermo-electric disturbance. 



Now, in imagination, let us take away both the observer at 

 the circle and the one at the galvanometer, and in the latter 

 case remove the scale also, and put in its stead a photographic- 

 ally sensitive plate. As the needle swings to the right or left 

 the spot of light will trace upon the plate a black horizontal 

 line, whose length will show how far the needle moves and 

 how great the heat is which originated the impulse. If this 

 be all, when under an impulse originated by the movement 

 of the spectrum over the bolometer thread the needle swings 

 a second time, it will go over the same place; but if the plate 

 have given it by clockwork a uniform vertical movement pro- 

 portional to the horizontal movement of the spectrum, the 

 combination of the two motions of the needle and the plate 

 will write upon the latter a sinuous curve which will be, in 

 theory at least, the same as the curve formerly deduci- 

 ble, only with much pains, from thousands of galvanometer 

 readings. 



If we suppose that the movements of the invisible spec- 

 trum, as well as of the plate, are controlled by the same 

 clockwork, so that this spectrum is caused to move uniformly 

 over the bolometer thread, and that these movements are, by 

 accurate mechanism, rendered absolutely synchronous with 

 those of the moving plate, it is clear that we shall be able 

 to readily deduce from the photographic curve traced on the 

 latter not merely the amount of the heat, but each particular 

 position in the spectrum of the thread of the bolometer, 



