936 
MR. A. E. TUTTON ON AN INSTRUMENT FOR PRODUCING 
so that the goniometer may be approached closely to the monochromatic light apparatus 
without leaving an inconvenient amount of the lower base projecting towards the 
observer. After completion of the above preliminaries the goniometer is rotated into 
the position shown in the illustration, when the axis of the collimator of the gonio¬ 
meter forms a continuation of that of the exit-tube of the monochromatic lififht 
o 
apparatus, and the illumination tube passes through the smaller aperture in the 
screen and enters the diffusing tube, its objective nearly touching the ground-glass 
screen. The more finely-ground screen affords the best illumination of the Websky 
slit. The goniometer lamp is used during the observations of minimum deviation in 
order to illuminate the verniers, which it does very brilliantly when the door of the 
aperture is temporarily opened. 
Upon generating the light in the lantern, setting the prism circle to the reading 
recorded for light of the first wave-length to be employed, and observing through 
the telescope of the goniometer, the two images of the Websky slit (supposing the 
crystal to be bi-refriugent) in the colour corresponding to the desired wave-length, and 
corresponding to the two indices of refraction afforded by the particular prism, will 
be observed in the field of the telescope. These images are then to be accurately 
arranged for minimum deviation, brought to the cross-wire by movement of the 
telescope, and the readings of the goniometer circle taken in the usual manner. In 
order to determine the direction of vibration of the rays corresponding to these 
refracted images it is usual to interpose a nicol prism somewhere in the path of the 
ray. Such a nicol is supplied with the goniometer for insertion in the illuminating 
tube : it is more convenient, however, to employ it as an adjunct of the telescope, 
placed in front of the eye-piece. It can then be readily removed if it is desired to 
observe both images in the field at once, or to avoid loss of light while placing the 
images ; it need only be employed in order to determine their planes of vibration, 
and to extinguish each in turn while placing the other to the cross-wire in cases of 
feeble double refraction when the images are so close as to almost or quite overlap. 
Such a nicol is supplied by Fuess for use with the Liebisch total-refiectometer; it is 
mounted in front of a telescope similar in power to the one most frequently employed 
for goniometrical work, but fitted with a silvered indicating circle, against which the 
graduated circle of the nicol rotates, and the nicol itself is constructed with fiat ends, 
which pass more light. 
The brightness of the images obtained by use of the monochromatic light apparatus 
now described is far superior to that obtained by illuminating the slit with coloured 
flames, and the images observed with C, F, and G light are immensely brighter than 
those aflbrded by the use of a hydrogen Geissler tube. Of course the actual bright¬ 
ness depends upon the individual crystal prism with which the observations are being 
carried out. In case the practice is followed of increasing the transparency of the 
prisms and obliterating any slight distortion of the fixces by cementing thin glass 
plates over the faces, with a solution of balsam in benzene, the definition and bright- 
