938 
MR. A. E. TUTTON ON AN INSTRUMENT FOR PRODUCING 
convenient distance of the prism from the slit is about eight inches. The diffusing 
tube is removed from in front of the exit slit, and the direct light from the latter 
may then be concentrated upon the ground face of the prism by means of a condensing 
lens of three or four inches focus, an ordinary microscope condenser on stand fitted 
with bail and socket joint serving admii'ably. The illumination is much greater than 
when the condensed light from coloured flames is employed, and the limiting line of 
total reflection can be more readily adjusted to the cross-wire of the telescope. The 
latter is accurately adjusted for parallel rays, and carries a nicol prism in front of the 
eye-piece in order to extinguish one of the limits in the case of doubly refracting 
substances, while rendering the other limiting line more distinct. Care must be 
takeii to avoid the admission of light into the objective of the telescope other than 
that which emanates from the surface of contact between crystal and prism, in order 
that the limiting line shall be as distinct as possible. For this purpose the small tube 
supplied with the instrument, which adapts on to the objective end of the telescope, 
is used. This tube is made to terminate, as close to the crystal as is compatible with 
tlie necessary amount of rotation of the total-reflectometer, in a cap pierced by a 
rectangular aperture of suitable small size. As the illumination by the monochromatic 
light apparatus now described is so good, the author uses a cap with an aperture only 
half the size of the smallest supplied by Fuess, namely, about one-and-a-half by one 
millimetre, thus being quite certain of the exclusion of other light than that from 
even a small crystal. By moving the condenser after finding the limiting line, so 
as to alter the angle of incidence, a position will be found for which the greatest 
difference of illumination on the two sides of the line is apparent, when it may be 
most accurately brought to the cross-wire. 
The adjustment of the crystal upon the total-reflectometer, and of the prism of the 
latter with respect to the goniometer, and also the determination of the position of 
the normal to the face of the prism from whicli the totally reflected light emerges, 
should first be carried out in white light in the manner described by Liebisch,* 
employing the goniometer lamp. 
The determination of the angle or angles of total reflection is then carried out by 
bringing the limiting line or lines of total reflection to the cross-wire of the telescope, 
employing light of each desired wave-length in turn. During these operations the 
circle is maintained clamped to the crystal carrier as it was when determining the 
position of the normal to the prism face, in order to be able to ascertain the angular 
difference between that position and the direction of the various limits of total 
reflection, from which, together with the knowledge of the angle and refractive indices 
of the prism, the required angles of total reflection can be calculated. Having taken 
two sets of such observations if the crystal is bi-refringeut, with the aid of the nicol, 
for the two limiting lines corresponding to the first desired position of the crystal, 
the whole series are repeated with the crystal rotated in its own plane by means of 
* ‘ Zeitschrift fiir Insfci'unientenkunde,’ 1884, 185, aiid 1885, 13. 
