43 2 The Smithsonian Institution 



rest that is visible. The actions, then, to which nearly all 

 the changes on earth are due, go on principally in this invisi- 

 ble region ; but, with the exception of some investigations by 

 Draper and Becquerel in the part just below the visible red, 

 this was all that was known in the matter twenty years ago ; 

 for since these rays cannot be seen, and cannot be made evi- 

 dent by ordinary photography, there remains no way of in- 

 vestigating this most important region, except by means of 

 some instrument which, like the thermometer or the thermo- 

 pile, will register the heat. For lack, then, of a more sensitive 

 instrument than science possessed, in this way, very little had 

 been done until the year 1881, in which the writer invented 

 a more delicate method of measuring heat, by means of 

 an instrument which he called the " bolometer." This 

 consists essentially of a metallic tape, usually about a 

 third of an inch long, but narrower and far thinner than a 

 human hair, through which an electric current is kept con- 

 stantly passing. It is found that the slightest change in the 

 heat which falls on this tape will affect a distant galvanometer 

 connected with it, so that as the effects of vision are no way 

 concerned, but only of heat, this may be compared, figura- 

 tively, to an eye which sees in the dark. Moreover, as this 

 thread can also be pointed with extreme precision, as in the 

 case of a vertical thread of an ordinary transit instrument, 

 the greater sensitiveness is accompanied by a corresponding 

 accuracy of measurement. 



This instrument was, at that time, able to indicate a change 

 of temperature of one one-hundred-thousandth of a degree, 

 and it had the incidental advantage that it could be pointed 

 so as to tell, within a fraction of a minute of an arc, in what 

 part of the spectrum the change to which it was sensitive was 

 found. 



A full description of the bolometer must be sought else- 



