REPORT OF THE SECRETARY. 47 



small amounts of heat, but it now sees with these what neither 

 the eye nor the photograph can sec. When the writer took 

 charge of the Astrophysieal Observatory of the Smithsonian 

 Institution the bolometer, with its attendant galvanometer, 

 eould recognize a change of temperature of less than one- 

 hundred-thousandth of one degree Centigrade. With the 

 changes which he and others have since introduced in the instru- 

 ment and its attendant galvanometer, it can now recognize less 

 than one one-hundred-millionth of one degree. As much as 

 a thousandfold gain in sensitiveness has, then, been attained 

 over the former conditions, and a manifold further increase is 

 hoped for by the use of the more sensitive galvanometer 

 now T being developed under the immediate care of Mr. Abbot, 

 the Aid Acting in Charge. 



Even with this remarkable progress the bolometer is still 

 far less sensitive than the eye in its capacity to detect radia- 

 tions of wave-lengths suitable for eye observations, but, as is 

 well known, it has the great advantage that all rays affect it 

 equally, whether visible or not, and that hence it can see 

 where the eye can not. 



In this little and inadequately installed Smithsonian Obser- 

 vatory the bolometer has extended the known spectrum to a 

 wave-length many times that known to Sir Isaac Newton, and 

 its use has spread from this country to every physical labora- 

 tory in the world where such researches are carried on. It is 

 growing more sensitive each year with continued improve- 

 ments, to which there seems to be no assignable limit, and 

 its future promises to be as full of value as its past. 



The urban situation of the Observatory puts serious diffi- 

 culties in the way of investigations which, like the one just 

 referred to, require exceptional steadiness and freedom from 

 magnetic fluctuations. An astrophysieal observatory should 

 evidently be located where smoke, lights, noise, traffic, and 

 heavy electric currents are at a distance. That the Smith- 

 sonian Observatory should still, after twelve years, be in its 

 present situation and with merely temporary w r ooden build- 

 ings for its home is indeed far from the expectations cherished 

 at its inception, a condition of affairs which the Secretary still 

 ventures to hope will be changed. 



