S. P. 



Langley — Energy 

 Table II. 



and T 



r ision. 



367 



nate deviations 



and reducing factors (tangents) 

 vave-lengths for great Hilger prisrr 



corresponding 



to adopted 



Wave-lengths. 



Adopted 

 Tangents to curve. 





Deviations. 





•35 



2-28 





48°00' 





•38 



1-94 





47 10 





•40 



1-73 





46 42 





•45 



1-27 





45 42 





•50 



•88 





44 58 





•55 



•62 





44 28 





•60 



•46 





44 07 





•65 



•36 





43 50 





•70 



•30 





43 38 





•75 



•27 





43 26 



•768 



•26 





43 22 





Table III. 



Co-efficients of reflections from two surfaces of silver. 



Wave-lengths -35 -38 -40 "45 -50 -55 -60 '65 '70 "75 

 fleeted from two 



•79 -82 -845 -86 "875 -885 



Percentage reflected from two 



surfaces ... *37 *54 '63 '73 

 Reduction factor 



(reciprocal) 2'70 1*85 1-59 1'37 1'27 1*22 1-18 V16 1*14 1-13 



Table III is a table for the selective absorption of silver 

 referred to such a lamina as is spread by the Martin process on 

 the front surface of the glass in its ordinary application. It is 

 prepared from unpublished observations made by the writer 

 with the bolometer in the course of the year 1881, and for 

 the method of its preparation the reader is referred to the foot 

 note.* It will be seen from this table that while such a silver 

 film exercises a considerable selective absorption in the ultra 

 violet and even at the blue end, it exercises less as the wave- 

 length increases, and in fact an extension of it would show a 

 still enhanced power of reflection for infra red rays. It is with 

 these infra-red rays that our measures in previous researches on 

 radiant heat at this observatory have been hitherto mainly 

 made. Accordingly our measures of the selective reflection in 

 the ultra violet, to which we have given comparatively little 

 study > have not been repeated with all the care which the subject 

 deserves, and we recommend a more complete determination 



* The selective absorption of silver has been deduced by bolometric measure- 

 ments in the solar spectrum, with a Rutherfurd grating, by producing multiplied 

 successive reflections of the light from silver before allowing it to enter 'the silt 

 of the spectroscope and determining successively the variation in the intensity 

 of different rays according to the number of reflections. The observations are 

 reduced by a logarithmic formula. 



