The Astrophysical Observatory 433 



where ; but, in further explanation, it may be said that the 

 electric current always passing flows less freely when the 

 minutest degree of heat falls upon the strip, and more freely 

 when this is made in the least colder, so that the galvanome- 

 ter needle swings in the first case to the right, and in the 

 second to the left ; and this, at present, may be arranged to 

 record changes of temperature as small as one millionth of a 

 degree. When this minute strip, or tape, is moved through 

 the invisible spectrum, the tape being parallel to the Fraun- 

 hofer lines, since what is black to the eye is cold to it, its 

 contact with one of them produces cold, which increases the 

 flow of electricity, and the galvanometer needle moves as 

 described. When it passes into a warmer region, the needle 

 moves in the opposite direction; and in each case the amount 

 by which the needle moves is proportional to the degree of 

 heat or cold in question, so that the final result is the same 

 as if a thermometer could be constructed much finer than a 

 human hair, from which all of these indications could be read 

 on such an extended scale that the millionth part of a degree 

 was visible, this thermometer being moved through the spec- 

 trum, and falling or rising, according as it meets one of these 

 dark and cold lines or goes into a warmer region. This 

 rise or fall indicates, then, the presence of such a line, whe- 

 ther the eye can see it or not, and when we pass out of the 

 visible into the invisible region, this method remains trust- 

 worthy where the eye and photography both fail us. 



When the instrument was first used, at least two observers 

 were required, one to note the reading of the circle which 

 fixed the place of the bolometer in the spectrum, and another 

 who sat at the galvanometer and noted through how many 

 divisions of the scale the needle swung, owing to the electric 

 disturbances, the whole process being comparable to a groping 

 in the dark, involving going over and over the work again 



