326 Prof. D. B. Brace on Double Refraction in 
described. It was found impossible to obtain a satisfactory 
match with the half-shade system alone without some other 
compensation. On examining with the analyser, vertical 
bands (they appeared horizontal of course) were seen which 
were quite regular and symmetrical on each side of a central 
black band. The distance apart of these bands and their 
distinctness were less as the number of7internal reflexions 
were increased. The total number of passages through the 
prism varied from ten to eighteen, or a total distance in the 
glass of from 420 cms. to 756 cms. An attempt was made to 
compensate by means of the horizontal strip of glass with the 
clamp for producing flexure. This system was mounted on 
a universal system so as to bring the black band in the glass 
strip vertically in front of the analyser. In one position, 
under flexure, the resulting bands became narrower. On 
reversing it, so as to interchange the compressed and dilated 
portions of the strip, the bands became wider and finally 
disappeared as the flexure was increased up to a certain 
point. They then reappeared and became narrower with 
increasing flexure. This compensation increased the sensi- 
bility so that a moderate pinch of the prism by the fingers 
gave a marked shift of the central band which could be 
observed by means of the two cross-wires already referred 
to. However, the compensation was not sufticiently satis- 
tactory to obtain good matches with the half-shade. Further- 
more, the slightest flexure of the trough or deviation of the 
beam of sunlight caused the “ match” to change in the one 
case or the band to shift in the other, owing to the narrowness 
of the beam. Observations with this arrangement would thus 
be likely to prove unreliable, and the system was finally given 
up for another which could be rendered more stable, optically, 
and in which artificial light could be used. 
T'wo cylinders of flint-glass, each 22°3cms. long and 2°4cms. 
in diameter, and of mean index np =1°77, were mounted on 
adjustabie supports between the polarizing and analysing 
systems. The former consisted of the nicol and half-shade 
system used previously as the analysing system. ‘This was 
observed through the analysing nicol by a low-power tele- 
scope. The source of light was an acetylene flame into 
which was injected, broadsides, a flat stream of oxygen 
through a fish-tail burner. This increased the brightness of 
the field of view several times, and extended the sensibility 
of the settings by a corresponding amount. ‘The entire 
system was mounted on a common base, so that once a match 
was obtained it could be moved without disturbing it. These 
cylinders were especially well annealed Jena glass used a 
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