36 
as applied to the Dispersion of Light), and by Dale and 
Gladstone, were made with the instrument described in the 
Proceedings of the British Association for 1839. It differs 
from the instruments usually employed in researches of this 
kind in having the circle vertical instead of being horizontal. 
This arrangement interferes with the possibility of measuring 
a double deviation, and renders the telescope liable to flexure ; 
the absolute results are consequently less deserving of con- 
fidence than those which are comparative. Van der Willigen’s 
observations are given in Poggendorff’s Annalen, B, 122, 8.191. 
Observer. Temp. C. Index for D at t® C. Index at 0° C. 
Haasan h(n ene aceenes 1875 LES SSE 133449 
Brash Ofer ae: possess Se si5 IESE Tie 1:33449 
Hrawnborer <csc<ce. ste 10 1:33358 1°33390 
HrauMhGOler ies ccees cx fos 11°56 133359 1:33399 
Bowel sent ek act ee 158 13343 3350 
Dale and Gladstone 0 13330 1:3330 
Van der Willigen ... 16°58 133332 133406 
Van der Willigen ... 22°37 133282 153400 
Mean 1°33412. 
But the index for D out of ice into water is 101952. Hence 
the index for D out of air into ice is 130858. The value 
of this index found by Dale and Gladstone is 13089. Fraun- 
hofer estimates the place of the brightest part of the solar 
spectrum to be at a distance from D equal to one-third or 
one-fourth of the distance between the lines D, #. Taking 
this distance equal to (029) DH = (0-062) DH, and observing 
that according to Dale and Gladstone the index of refraction 
of water at 0° for the line H is 0:0108 greater than for the 
line D, the probable index of refraction of ice for the brightest 
part of the solar spectrum will be 1°30925. The value 1°3089 
of the index of ice for the line D gives for the mdex of the 
brightest part of the spectrum 13096. These values agree 
