362 S. P. Langley — Energy and Vision. 



Description of the Apparatus. 



The measures have all been made in a dark room from which 

 every source of outside light is excluded except that which 

 enters the slit of the spectroscope. 



The light from the siderostat mirror, M (fig. 1) passes through 

 a small aperture in the north wall and falls on the slit (s x ), 

 (which has doubly moving jaws, 34 mm high, set in these experi 

 ments at a standard distance of 0'l mm ), then on the great colli- 

 mating lens (I) of 755 cm focus (aperture ll*9 cm ), t x being a paper 

 tube to prevent the lateral diffusion of light from dust parti- 

 cles, p is a glass prism,* m, the concave mirror of 148 cm focus, 

 which here forms upon a second slit (s 2 ) a spectrum about 7 mm 

 high and 90 mm long in the easily visible part from A to H. 

 The prism and mirror are mounted on the spectro-bolometer 

 already elsewhere described,f and which is provided with a 

 circle reading to 10" of arc. By setting this circle, any color 

 can be brought on the slit (s a ). The light which the mirror 

 has converged into that part of the spectrum overlying this 

 slit passes through it, diverges and falls upon a black paper, 

 figure 2, in which is a central aperture l cm square, occupied by 

 part of a table of logarithms, printed in small black type on 

 white paper. This table can be adjusted to bring different 

 figures in view, but is otherwise fixed relatively to the black 

 paper screen which (with this central centimeter occupied by 

 figures) is mounted on a slider. The rod (r) on which the 

 slider moves is a prolongation of the spectroscope arm, made 

 of a light wooden rod graduated so that one can read the posi- 

 tion of the slider to a centimeter by feeling of notches in the 

 dark. The zero of this rod is at slit 2 on which the spectrum 

 is thrown. 



It is to be observed that it is necessary that the square of 

 figures should be small in order that it may be slid nearly to 

 the apex of the cone of light and remain covered thereby. 



It is to be noted also that at a constant distance and in a 

 feeble light, these small figures may be invisible to the naked 

 eye and most distinctly visible to the same eye with a magnify- 

 ing glass. For two eyes of different foci, the amount of 

 light with which the same figures will be read will probably 

 vary. It follows that even if the same person read from 

 beginning to end of the series, his readings will not be com- 

 mits principal constants are : height of face 1 l-5 cn \ width, 10'5 cm , while for a tem- 

 perature of 28° C. the refracting angle is 60° 06' 45", deviation 

 H = 46° 45' 85" 

 &! = 44 45 55 

 D 2 =44 11 15 

 A = 43 24 05 

 « 2 ("little Omega") = 41 3-1. 



f "Researches on Solar Heat," Prof. Papers of the Sig. Serv., No. 15, p. 130. 



