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



The work extended over an arc of about 19°, with the following 

 results : — 



For one wave the agreement was close throughout. This section 

 differed but slightly from the circular section of radius 1'53013. 



For the other the differences were much greater. 



The results of experiment were represented by a curve, which in 

 the neighbourhood of the lesser axis of Fresnel's section lies within 

 that section, cutting it at about the middle of the arc considered, and 

 afterwards lying without it. 



The excess of experiment over theory changes in the arc considered 

 from 



-•0002 to +-0005. 



This agrees with the result for the first prism in lying outside Fresnel's 

 surface as we approach the major axis. 



I then proceeded to estimate the effect of any possible errors made 

 in the determination of a, b, c, or the position of the plane, and showed 

 that no change at all within the limits of experimental error would 

 reconcile theory and experiment more closely. As a test of the accu- 

 racy of the experimental work, it may be stated that a series of obser- 

 vations, taken at an interval of some three months previously to those 

 described above, gave results which rarely differed from those results 

 by more than 



•00004. 



To proceed now to the second crystal. The measurements on it 

 were made at an interval of nearly a year after those already described. 

 The crystal was in the form of a hexagonal prism, the base of the 

 prism being nearly perpendicular to c. This base was polished. The 

 other end was cut so as to be inclined to the base at an angle of 

 35° 2' 56", the line of intersection of the faces of the prism thus 

 formed being nearly parallel to that of m and e. One of the faces, m, 

 was cut so as to be inclined to the oblique section at about 37°, the 

 line of junction being again nearly parallel to that of m and c. So that 

 I thus formed two prisms whose principal planes were nearly coincident, 

 having one face in common. 



By this means I was able to work over an arc which extended from 

 the neighbourhood of the principal section AOC to more than 70° on 

 the other side of it. 



The planes cut the principal plane, AOC, in two points, L I/, such 

 that — 



