468 REPORT — 1894. 



and the one at the galvanometer, and in the latter case remove the scale 

 also, and put in its stead a photographically 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 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 deducible, only with such pains, from thousands of such galvano- 

 meter readings. 



If we suppose that the movements of the invisible spectrum are con- 

 trolled by 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 photo- 

 graphic 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, 

 which alone can correspond with any given inflection of the curve. 



Thus simple is the theory, but no one had better occasion to know how 

 difficult the practice would be than myself. 



The researches by the old method and the early attempts to improve 

 them were interrupted by my acceptance, in 1887, of a position which 

 implies the administrative charge of different branches of the public 

 scientific service, and of duties largely incompatible with original re- 

 search. What time could be spared from these was, however, partly 

 employed in elaborating the plan of investigation just referred to. An 

 appropriation had been asked of Government for the establishment, on a 

 modest scale, of an Astro-physical Observatory in Washington, whose first 

 work should be the investigation of the whole infra-red solar spectrum, by 

 some means which would open that great region to knowledge. It had 

 been asked of Government, because it seemed that such knowledge, if 

 attained, might teach us facts about the sun and the absorption of its rays 

 by the terrestrial atmosphere, which, there was ground to hope, would 

 ultimately lead to results of such importance as to justify this national aid. 



These observations were resumed in 1890, on the new system, with the 

 aid of the Smithsonian Institution which provided larger and more 

 efficient apparatus, whose design embodied the results of nearly fifteen 

 years' study of these subjects. 



Pending the provision of a suitable observatory building, an inadequate 

 and temporary one was erected in the Smithsonian Park in Washington, 

 to shelter the apparatus presently to be mentioned, with which it was 

 designed to commence work, while making provision for more permanent 

 scientific quarters — which, I may add, are still lacking. 



The Foucault Siderostat — perhaps the most powerful instrument of the 

 kind existing — was originally made by Sir Howard Grubbof Dublin, from 

 my indications ; but its dispositions have since been considerably modified. 

 A beam from its twenty-inch mirror is conveyed through the slit of a 

 horizontal collimating telescope having a rock-salt objective of nearly seven- 



