284 
DR. T. M. LOWRY: NATURAL AND MAGNETIC ROTATORY DISPERSION 
6. Experimental Methods. 
1. The polarinieter, a triple-field instrument of the Lippich pattern, made by the 
Cambridge Scientific Instrument Company, was graduated to read by means of a 
vernier to 0°‘002 ; but in practice readings were usually taken only to 0°‘01, without 
using the vernier. Whilst it may be possible under some conditions to read a 
polarinieter to the third decimal of a degree, it is only very rarely that such readings 
have any absolute value; in the present investigation the policy was deliberately 
adopted of making the readings so large that any impurities in the light or faults in 
the material should obtrude themselves to such an extent as to make the readinrf of 
the rotations impossible; relative values based upon minute readings of smaller 
rotations would have been of much less value, since most of the sources of error would 
have been overlooked. The aperture of the triple field was 6 mm., the main polarising 
and analysing Nicols being 10 x 10 x 30 mm. 
2. The spectroscopes and prisms were by Hilger and need not be described in detail. 
The slits were arranged to open symmetrically ; the slit of the spectroscope could be 
opened to a width of 3 mm. giving an image 6 mm. in width; the slit of the 
polarinieter could be opened to the full width (6 mm.) of the aperture in front of the 
triple field. The constant-deviation spectroscope was provided with a drum which 
could be calibrated in wave-lengths, so as to illuminate the polarimeter with light of 
any desired colour selected from a continuous spectrum ; but it is impossible to 
recommend the use of such a method, as the wave-length of the light actually 
delivered to the slit of the polarimeter was found to be seriously aflbcted when the 
half-shadow angle was altered. This displacement had not been anticipated when 
using flat-ended Nicols prisms but is a necessary consequence of the method followed 
in constructing them; it was discovered when using narrow lines to illuminate a 
narrow slit, by noticing that the line was displaced and partially obliterated on 
changing the sign of the half-shadow angle. This observation was regarded as a 
further justification of the policy of using monochromatic or multichromatic, instead 
of continuous, sources of light; the only effect of the displacement was then a slight 
narrowing of the line, which could be overcome by readjusting the drum of the spectro¬ 
scope ; with a'continuous spectrum, the wave-length of the light would be altered and 
the readings would be rendered uncertain from a cause that would not easily have been 
detected or traced. 
3. The setting of the quartz was effected with the help of a reflecting eye-piece. 
The pointer of the eye-piece was brought to coincide with the centre of the image .of 
the slit of the ^spectroscope; the telescope was then racked in till focussed for infinity 
and the bright image of the pointer, reflected from the surface of the quartz, was brought 
point to point with the dark image of the pointer itself. Tlie setting was effected in 
a very simple way: the water-jacket containing the quartz cylinders rested in a" 
rectangular trough ; by placing thin cards on one side or other of the trough, the 
quartz could be tilted in two planes at right^angles to one another ; the cards were 
