3G2 
LORD RAYLEIGH OH THE CONSTANT OE MAGNETIC 
In these formulae /3 may he chosen at pleasure by suitable adjustments of the 
polarising arrangements. In order to get the best result, cffi must be made a 
maximum by variation of j3. a and /3 0 being treated as constants. The maximum 
occurs when (3=/3 0 , and its value in the last case is 
hE = A 
2 a 
Jo 
Taking hE/A=- 2 h-, wlrich is probably about as small as can be expected in practice, 
we have for the least perceptible value of a 
/3 
o 
whereas without the half-shade arrangement, and with a Nicol simply set to extinc¬ 
tion of the original light, 
1i(P 
Ifj 
1 
: 25’ 
so that 
According to these numbers the half-shade arrangement would have a tenfold 
superiority, a result not fully borne out in practice. In explanation of this it is 
important to notice that the procedure in the absence of a half-shade arrangement 
would in reality be very different from what we have tacitly supposed. The expe¬ 
rienced operator, in setting a Nicol to the position of maximum extinction, does not 
judge merely by the degree of darkness attained in the final position, but displacing 
the analyser alternately in opposite directions, estimates the position which lies mid¬ 
way between those which give similar revivals of light on the two sides; or, 
endeavouring to retain in his memory a certain degree of brightness, he may take 
actual readings on both sides, of which the mean will correspond to the desired 
position. In this way the fundamental advantage of the half-shade method is in a 
sense attained, the only difference being that the brightnesses to be compared are seen 
consecutively after a short interval of time, instead of almost simultaneously; and 
even this difference becomes less important when the line dividing the field of view 
of the half-shade apparatus is so coarse that it cannot be rendered invisible. 
The carrying out of this method is facilitated by a device which is worthy of trial. 
The Nicol may be mounted loosely, so as to be capable of turning through a small 
angle (2 or 3 degrees) between two stops. These stops are rigidly attached to a 
rotating piece carrying the vernier, and it is to the position of this piece (and not that 
of the Nicol) to winch the readings relate. In taking an observation the piece is 
turned until the degree of brightness is unaltered, when the Nicol is put over from 
the one stop to the other. It is probable that under these advantageous conditions 
