12 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 92 



taken by this instrument on Oct. 7, 1880, used with an extremely 

 delicate reflecting galvanometer by Elliot, of about 20 ohms resistance 

 and a reflecting grating on speculum metal by Mr. Rutherford of 681 

 lines to the millimetre. Measures have been taken every fair day since, 

 the source of energy being the sun. 



.... The ' Balance ' then, whose acting face is only about 1/30 

 the length of the visible spectrum, and less than i/ioo the length 

 within which energy is found in a degree sufficient for it to measure, 

 receives nearly homogeneous rays (which have passed through no 

 absorbing medium whatever except the solar and terrestrial atmos- 

 pheres) , and this extremely minute amount of heat is found to give a 

 galvanometer deflection of some hundred divisions, where thermo- 

 piles have hitherto failed to register any (on homogeneous rays). 



". . . . They are hitherto unpublished, and they at least, though as 

 yet approximate, show that the Jicat ma.vimmn in a normal spectrum 

 is not in the ultra-red, but is at least as far up the spectrum as the 

 orange near D ; and this result may be relied on, any smaller values 

 below A =.0007, as well as all favorable atmospheric circumstances 

 (high sun, blue sky, etc.), rather tending to move it toward the 

 violet." 



"ON THE AMOUNT OF THE ATMOSPHERIC ABSORPTION" 



" Let us first suppose the radiation of the heavenly body to be 

 really composed before absorption of two portions, A and B. Let A 

 have a special coefficient of transmission (a), and B another, special 

 to itself {h). Then, if we assume (still for considerations of con- 

 venience only) that each of these portions, is, separately considered, 

 homogeneous, we may write down the results in the form of two geo- 

 metrical progressions, thus : 



Table i 



