302 E.C. Pickering on different forms of the Spectroscope. 
r=5, siné=nsin>.....(1). If dn be the difference of the 
indices of refraction of the two rays, di will be their angular 
Fo fo, 
divergence. Differentiating (1), di= —2 dn=- tani dn.. (2), 
in which = tang 7% serves as a measure of the dispersion under 
different angles of incidence. It differs essentially (when the 
angle of incidence is large) from the deviation which is com- 
monly, but incorrectly, assumed as the measure. 
Comparative dispersion and deviation of a ray entering a medium in which n=1'5. 
Angle of inci- 
, 0°; 15°} 30° | 45° [5691971 60° | 75° | go0° | 85° | 90° 
"0 | -179) -385 | -667 | 1-000 | 1-1565) 2-488 | 3-781 | 7-620 | © 
ae - m 0 
Deviation ir, 0°'5° 4/110 94,10" 00 (24° 00' | £4" 44 | 04” 00/55 DO |43> 40 40 ll 
as dispersion, |"0 ' -231| -465 | ‘746 | 1-000 | 1-093 | 1°543 | 1-720 | 1-917 | 2-121 
The dispersion then increases much more rapidly than the 
deviation ; hence in spectroscopes whose deviation is the same, 
that one will disperse most, in which 7 and therefore « is the 
greatest. 
ut in reality the light so 
in this state another la 
the amount reflected. Fresnel showed that of a ray 
z pees ae 
aes ae while a ray polarized in a plane perpendicu- 
