346 
LORD RAYLEIGH ON THE CONSTANT OF MAGNETIC 
field into two parts. The upper half of the field is thus rotated by a thickness of 
syrop equal to the entire width of the cell (say ■§• inch), but in the lower half of the 
field part of the thickness of syrop is replaced by glass, and the rotation is corres¬ 
pondingly less. With a pretty strong syrop a difference of 2° may be obtained with 
a glass inch thick. For the best results the operating boundary should be a true 
plane nearly perpendicular to the face. The pieces used by me, however, were not 
worked, being simply cut with a diamond from thick plate glass ; and there was usually 
no difficulty in finding a part of the edge sufficiently flat for the purpose, i.e., capable 
of exhibiting a field of view T sharply divided into two parts. I had expected to be 
troubled with depolarisation, especially in the thick glass, but a small piece thus cut 
out of a large plate is relieved from most of the strain to which it w r as originally 
subject. Probably more care would be required in experiments where a strong white 
light could be used; but by previously testing the rather thin plates used for the 
sugar cell and for closing the CS 3 tube, I was able to secure a field of view either 
half of which under the actual circumstances could be made quite dark by suitable 
orientation of the analysing Nicol. 
By this use of sugar half-shade polarimeters may be made of large dimensions at 
short notice and at very little cost. The syrop should be filtered (hot) through paper, 
and the cell must be closed to prevent evaporation. 
7. On leaving the sugar cell the light entered the column of bisulphide of carbon (H). 
To contain the liquid two tubes of brass were employed at various times, the ends 
being closed with plates of worked glass cemented to the metal with a mixture of glue 
and treacle. Near one end these tubes were provided with a lateral (vertical) branch, 
closed with a cork, through which passed the stem of the thermometer used for 
observing the temperature of the CS 3 . The length of the larger tube (used in 
Series I. and II.) was 31’591 inches, and the diameter about If inch. The length 
of the smaller tube (used in Series III.) was 29765 inches, and the diameter 1 inch. 
When, as in Series I., it was wished to cause the light to traverse the tube more 
than once, mirrors were necessary at the ends of the tube. They consisted of plates 
of thin looking-glass, from which part of the silvering was removed, and by means of 
a little glycerine they were brought into optical contact with the plates by which the 
tube was closed. This arrangement was simple, and had the further advantage of 
practically annulling some troublesome reflections ; but the want of means of adjust¬ 
ment rendered it necessary that the closing plates should themselves be pretty 
accurately parallel. 
8. The internal diameter of the ebonite tube, upon which the helix was wound (§ 13), 
was about 1|- inch, and it was intended to utilise the annular space between the 
ebonite and the brass as a jacket, through which water at the temperature of the room 
might be made to circulate. This arrangement, however, failed utterly. Within 
about 10 minutes of the closing of the circuit of the helix, the definition was lost, and 
nothing further could be done until after a long interval of repose. The water-jacket 
