:( 328 ) 
Art. XXII. — Historical Account of Discoveries respecting the 
Double R^raction and Polarisation of Light. (Continued 
from Vol. IX. p. 152.) 
Sect. V. Account of the Experiments of Dr Wollaston. 
In the Bakerian Lecture for 1801, our learned and ingenious 
countryman Dr Thomas Young, pointed out the advantages of 
the Hugenian Theory of Light, in affording an explanation of 
several phenomena, which had not been accounted for by any 
other hypothesis. Dr Wollaston, who had invented a new me- 
thod of measuring the refractive powers of bodies, conceived the 
idea of employing this method to examine the accuracy of 
Huygen’s Theory of Double Befractlon. Huygens had himself 
done this by direct experiment ; but it was desirable to have 
the same examination repeated by a philosopher of Dr Wollas- 
ton’s accuracy, and by a method which promised to afford very 
nice results. In this way Dr Wollaston obtained the following 
results. Index of 
Refraction. 
1, When the line of sight bisects an acute angle of a na- 
tural surface of the spar, - - - - 1.488 
% When the plane of incidence is parallel to one of the 
sides, - 1.518 
3. In a direction at right angles with either side, - 1.537 
4. In the plane bisecting an obtuse angle, - - 1.571 
5. On a surface perpendicular to the axis of the rhomb, 1.488 
|6. The regular refraction found by the ordinary method, 
from an average of several experiments, - 1.657 
7. The inclination of two surfaces, . . - 105°.8' 
8. Inclination of a refracted perpendicular ray to the 
perpendicular, - - - - - 6°. 16' 
Having obtained these results, Dr Wollaston then contrasts 
them with those calculated 
by Haiiy. 
Observed, 
Calculated. 
Exp. 2. 
1.518 
1.6215 
3. 
1.537 
1.539 
4. 
1.571 
1.5736 
Observed angle, 
6°.16 
6».Ti 
