OF NITRO-CELLULOSE AND THE LAW OF ITS OPTICAL BEHAVIOUR. 
155 
The optical arrangements are modified for the new conditions as shown in the 
accompanying fig. 11. Light from the filament A of a Nernst lamp is focussed on to 
a vertical slit S by aid of a lens B, after passing through a Nicoi/s prism M and 
this narrow band is in turn focussed on the central section of the beam at D, and 
analysed by a second Nicol’s prism N. A lens E placed at a convenient distance 
from the beam transmits this light as a parallel beam to a reflecting prism F, from 
which it passes through prisms G, H. The spectrum so obtained is focussed on a 
glass screen L ruled with lines inch apart, and provided with a micrometer 
eye-piece for measuring the ordinates of the bands observed. 
The field of view consists therefore of the spectrum of a Nernst lamp filament to 
which is added the effect produced by a narrow section of a beam of rectangular cross- 
section under pure bending moment. The relative retardation, owing to this latter 
stress effect, produces black bands in the field having a variable distance apart 
depending on the optical law of the retardation of the wave-length. 
The general disposition of the field of view is shown in the accompanying fig. 12 in 
which bands of the first and second order appear on each side of the neutral 
axis C of the beam, and their co-ordinates are measured by reference to the 
graduations on the glass scale with the aid of a pair of parallel wires D, the positions 
of which can be adjusted vertically by a micrometer head E reading to eoVo inch, 
while complete turns of the screw are obtained from a scale F on the left, which also 
appears in the field of view. In order to calibrate the horizontal scale the Nernst 
lamp and nitro-cellulose beam are removed, the Nicols rotated to parallelism, and a 
beam of solar light focussed on to the slit. The position of lines of known wave- 
VOL. CCXXT.—A. 
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