494 
PROFESSOR HENRY A. MIERS; AN ENQUIRY INTO 
It was found that a perfectly definite reading being obtained with sodium light for 
total reflection—corresponding to the index of the saturated or slightly supeisatuiated 
solution—this reading was only slightly afiected when the crystal was moved up and 
even brought into actual contact with the prism face. The index does actually 
increase to a certain small extent as the crystal is brought nearer to the prism, and 
this is shown by Nos. 1-4 in Table II., p. 500. 
This indicates that tlie index of refraction only varies to a very slight extent, and 
tlierefore that the solution is only slightly sipiersaturated almost up to contact rvith 
the growing crystal. 
It is clear, however, that this experiment does not really determine the index of ■ 
the liquid in absolute contact with the crystal, whereas the ultimate layer is precisely 
that of wliich we require to know the composition, even if it be only a layer of 
inappreciable thickness. Is it not possible, however, to make use of the natural 
faces of the growing crystal as our totally reflecting prism ? This, if feasible, appears 
to be in fact the only method of deter- 
inimiip; the refractive index of this ultimate 
layer of the liquid. 
Preliminary experiments, in which the 
square g, 
nlass trough and an octahedron ol 
potash-alum were employed, presented 
considerable difficulties, but ultimately 
yielded a definite and clearly visible line of 
demarcation in the field of view, and 
showed tliat the method is a possible one. 
Let OAO'B (fig. 17) be the outline of four 
faces of an octahedron of alum adjusted 
upon the goniometer, and growing in the 
solution; T being the telescope and C the 
collimator. The angle between the tele-, 
scope and the collimator is first accurately 
measured by means of a glass plate mounted on the crystal holder, being twice the 
angle 0 between the position in which the glass plate refleccs ilie colliniatoi signal on 
to the cross-wires, and that in which the plate is perpendicular to the telescope as 
determined by the Gauss method. The front HK of the glass trough, HKNM, is 
then adjusted perpendicular to the telescope by the Gauss method, and the angle 
AOB is measured. A movable sodium flame at L is next adjusted by trial, and the 
crystal rotated, until the edge of tlie dark band indicating total internal reflection at 
OA is visible in the field of view and can be adjusted upon the vertical cross-viie. 
Let the reading for this purpose be (f >; then, if /3 be the reading for winch the 
image of the collimator signal is reflected upon tlie vertical wire from OB, and ^ + y 
the reading for which OB is perpendicular to T, then 13 -\- -y — (f> = 0 is the angle of 
