AND DISPERSION TN ICELAND SPAR 
427 
Column 1 gives the value of D -\-i, D being the observed deviation, and i the angle 
of the prism. (In the calculations D occurs only in the form L) i, therefore D-j- i 
is given in the tables instead of D.) Column 2 the observed angle of incidence. 
Column 3 the angle which the wave normal in the crystal makes with the normal to 
the faces of incidence calculated from the formulge 
cb' ^lr' 
tan ' - —tan 
cot 
(f) + y}r 
2 
tan 
1 
2 
Y 
already proved, and column 4 the values of /x or — calculated from 
Y sin $ sin -ip 
v sin (j> ’ sin \fr' 
On comparing the results for the ray C with theory I found so close an agreement 
that I thought it hardly requisite to work out all the calculations for the rays F 
and g. I therefore completed the calculations for only about a third of the obser¬ 
vations, giving a series of values of g in directions inclined at angles of about 4° to 
each other, extending in an almost continuous arc from the optic axis to directions 
perpendicular to it. 
These are contained in Tables Y. and YI. 
The middle column in each case gives the angle of incidence. The columns on the 
right refer to the ray g, those on the left to the ray F. 
For Table Y., Prism II., the results for the angle of incidence cfj have been calculated 
for the value 46° 36' 53" of the angle of the prism instead of 46° 36' 19" the value 
used for the results in which the angle of incidence is denoted by xfj. The reasons for 
this will be discussed in connexion with the theory. 
This closes the experimental part of the work. 
