498 



K. T. Glazebrook on Velocities of Normal [June 20, 



The work was carried out on two pieces of aragonite supplied by 

 A. Hilger, Tottenham Court Road. In both cases two of the faces 

 marked mm' (Miller's "Mineralogy," p. 567) gave the best reflexions, 

 and were therefore reserved to afford means for the determination of 

 the position of the artificial faces. We will consider the two separately. 



The first crystal was cut at Professor Stokes' suggestion, so as to 

 form two prisms. 



The edge of one of these was nearly parallel to the axis b of the 

 crystal, the mean axis of elasticity, so that the principal plane of the 

 prism almost coincided with the principal plane AOO of the wave 

 surface, and I was able to show that the error arising from supposing 

 the coincidence to be exact would never amount to as much as "00001. 



The axis OA of the crystal almost bisected the angle of the prism, 

 which was 42° 50' 30". The observations of deviation and incidence 

 were made with a goniometer by Grubb, lent me by Professor Stokes. 

 The circle, about a foot in cliameter, was graduated on silver, and was 

 read to 10" by verniers. 



Each observation was repeated two or three times on different occa- 

 sions, and the mean of the results taken. It was rarely that two 

 measures of the same quantity differed by 20". 



Careful precautions were taken to ensure the light passing in a 

 principal plane of the prism. 



The observations with this prism extended from about 8° on one 

 side of the axis OC to 16° on the other, passing almost through the 

 extremity of an optic axis. Observations were taken at angles of in- 

 cidence, increasing uniformly by 2°, thus forming an arithmetical 

 progression. 



The values of p for one wave were nearly constant, and varied but 

 little from 



1-68125. 



Let ii, b, c, be the principal refractive indices. The series of values 

 given above corresponds to the circle of radius b. So that 



b = 1-68125. 



The values corresponding to the other wave varied considerably. 

 According to Fresnel they ought to be radii vectores to an ellipse 

 axes a and c. 



a was determined by passing light along the axis OC, which wa. 

 possible, the crystal having been cut with this object in view. I found 



a = 1-68580. 



To find c recourse was had to the second prism, which had its edge 

 nearly parallel to the axis of c. I found 



c = 1-53013. 



