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



lateral diffusion of light from dust particles. ^ is a glass 

 prism*, m, the concave mirror of 148 centim. focus, which 

 here forms upon a second slit (sa) a spectrum about 7 millim. 

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

 H. The prism and mirror are mounted on the spectro- 

 bolometer already elsewhere describedf, and which is pro- 

 vided with a circle reading to 10'^ ot arc. By setting this 

 circle, any colour can be brought on the slit (§2). The light 

 which the mirror has converged into that part of the spec- 

 trum overlying this slit passes through it, diverges and falls 

 upon a black paper (fig. 2), in which is a central aperture 

 1 centim. 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 other- 

 wise fixed relatively to the black paper screen which (with this 

 central square centimetre occupied by figures) is mounted on 

 a slider. The rod {r) on which the slider moves is a prolon- 

 gation of the spectroscope arm, made of a light wooden rod 

 graduated so that one can read the position of the slider to a 

 centimetre 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 magni- 

 fying 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- 

 parable unless they are all taken under the same optical 

 conditions, e. g. all with the naked eye or all with glasses of a 

 certain strength. 



In these measurements a magnifying glass of 4*7 centim. 

 focus was used by all the observers, and in addition, two who 

 were near-sighted wore spectacles correcting this defect. 



* Its principal constants are : — lieiglit of face 11*5 centim., width 10*5 

 centim., while for a temperature of 28° C. the refracting angle is 60° 06' 46" ; 

 deviation : 



H =40 45 35 

 \ =44 45 55 

 D2=44 11 15 

 A =43 24 05 

 ©2 ("little Omega") = 41 34. 



t " Eesearches on Solar Heat," Prof. Papers of the Sig. Serv., No. 15, 

 p. 130. 



