44 The Smithsonian Institution 



As the strip of the bolometer passes through a dark line, its 

 temperature falls, less current passes through the cable con- 

 necting it with the distant galvanometer, shown at G, and the 

 needle of the latter instrument is deflected, showing how 

 great the radiant heat of the spectrum was at the precise 

 position in question. In the accompanying illustration are 

 shown two such curves obtained on different days, quite in- 

 dependently of each other, by two successive movements such 

 as have been described through the whole spectrum. They 

 represent nearly two hundred lines, which are otherwise 

 shown in the plate in the usual form as a line spectrum. 

 The general coincidence of the two curves one with another 

 affords the most convincing proof that could be desired of 

 the accuracy of the process, which thus obtains in a few 

 hours data which could hardly be obtained by a life-time of 

 assiduous labor with the old one. But this new method 

 is so sensitive that it can record more minute inflections 

 than are here set down, these inflections being intentionally 

 slurred over, as here given. 



When, however, we proceed with the aim of developing all 

 the minute deflections that are caused by the changes in the 

 atmosphere of the sun and the earth, we are confronted with 

 the difficulty common to delicate physical measurements of 

 every kind, that, owing to the sensitiveness of our apparatus, 

 it will register deflections due to causes which we are not 

 concerned with, and do not want to record. For instance, if 

 a wagon be passing in a distant street the jar communicated 

 to the ground, although quite imperceptible to all ordinary 

 sense, will be registered by the galvanometer, forming a 

 minute inflection of the curve, which might be confounded 

 with those produced by the action of the sun itself, the distant 

 sun and the wagon in the next street registering their action 

 in the same place and in the same manner. 



