470 KEPORT— 1894. 



across the bolometer is 5 metres in length ; and if the bolometer thread 

 be if\y of a millimetre in width, its angular value is evidently xTroVinr ^^ *^*^ 

 I'adius of the circle in which it moves, or a little over two seconds of arc. 

 When the heat is distributed over so large an area, that part of it which 

 falls on a thread of given diameter is of course proportionately less, so 

 that the greater precision of measurement demands a more sensitive con- 

 struction of the bolometer, as well as a more accurate mechanism for 

 pointing it. Improvements have accordingly been introduced in the con- 

 struction of the bolometer, and a need for greater sensitiveness in the 

 galvanometer has necessarily gone with them. This increased sensitive- 

 ness has caused increased liability in the latter to both systematic and 

 accidental perturbations, and the elimination of these has been found the 

 most formidable difficulty of the whole process. It has been effected largely 

 by placing the svhole apparatus under constant temperature conditions. 



I take pleasure in acknowledging the advantage I have found in 

 using both Mr. Boys's quartz threads, and the extremely small mirrors 

 which he, I think, first advocated in connection with the well-known form 

 of galvanometer due to Lord Kelvin. These and other collective improve- 

 ments made in the bolometer and in the galvanometer have now made the 

 former sensitive to changes of temperature in its striji which are demon- 

 strably less than t-otj^-oo-o ^^ ^ degree Centigrade. 



These are the principal pieces of apparatus, though I should mention 

 that a method has been found by which the very large salt prisms used 

 can be preserved in perfect polish while exposed to all the usual casualties 

 of observation. The actual prism in most frequent use was made from a 

 block of salt exhibited at the World's Fair by the Russian Government, 

 and presented to the Smithsonian Institution by its Commissioners. It 

 is about 18 cm., or over 7 English inches, in height. 



Before entering upon a description of the results obtained I desire 

 permission to speak of the aid I have received from the gentlemen whose 

 assistance I have been fortunate in securing. First to Dr. Hallock, then 

 to Professor Hutchins, Mr. Hubbard, and Mr. C. T. Child, and lately to 

 Mr. F. L. O. Wads worth and Mr. R. C. Child, the imprint of the labours 

 of the two latter gentlemen being upon almost all the details of the more 

 recent work. 



Residts. 



Let us recall that the infra-red spectrum from a rock-salt prism such 

 as that used is extremely contracted as compared with one from flint glass, 

 and still more contracted as compared with the wave-length scale. The 

 portion of the spectrum presented by photography reaches a little below 

 the band whose wave-length is about 1", and this was asserted by one of 

 the most eminent living authorities on the subject (Dr. John W. Draper) 

 when the Avriter commenced this work, fifteen years ago, to be the absolute 

 end of the heat spectrum. The writer has, however, since carried his 

 investigations by direct measurement to five or six times this wave-length, 

 and by indirect measurement much farther still, though what is now 

 exhibited does not go beyond a wave-length of about 4*". The invisible 

 heat spectrum of a 60° rock-salt prism through this great wave-length 

 includes only somewhat less than 2° of arc, ajid the first of these degrees 

 contains the greater proportion of the energy. 



On referring to the illustrations exhibited to the Association in 1882, 

 or even to later publications of results obtained by rock-salt prisms, though 



