10 



Prof. S. P. Langley on Energy and Vision. 



of the prism we must prepare a table or a graphic construc- 

 tion, deduced from an examination of the special prism 

 employed, showing the wave-length corresponding to the 

 position of minimum deviation of each raj. Figure 3 is such 

 a graphic construction derived from our own observations of 

 the constants of the prism employed, and Table II. gives the 

 approximate value of the tangents to the curve by means of 

 which we pass from the prismatic to the normal scale. 



Table II. 



Approximate deviations and reducing factors (tangents) cor- 

 responding to adopted wave-lengths for great Hilger 

 prism. 



Wave-lengtlis. 



Adopted. 



M 



TaEgents to curve 



•35 



2-28 



•38 



1-94 



•40 



1-73 



•45 



1-27 



•50 



•88 



•55 



•62 



•60 



•46 



•65 



•36 



•70 



•30 



•75 



•27 



•768 



•26 



Deviations 



o 



48 



00 



47 



10 



46 



42 



45 



42 



44 



58 



44 



28 



44 



07 



43 



50 



43 



38 



43 



26 



43 



22 



Table III. 

 Coefficients of Reflexions from two surfaces of Silver. 



Wave- lengths 



Percentage reflected from 



two surfaces 



Eeduction factor (reci- 

 procal) 



•35 



•37 

 2^70 



•38 



•54 



1-85 



•40 



•63 

 1-59 



•45 



•73 



•50 



•79 



r37 1-27 



•55 



•82 

 1-22 



•60 



•845 

 MS 



•65 



•86 

 1-16 



•70 



•875 

 1-14 



•75 



•885 

 M3 



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 It is prepared from unpublished observations 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 

 3'ear 1881, and for the method of its preparation the reader 



