430 Dr. Leeson on the Circular Polarization of Light 



ally be substituted. This analysing eye-piece fits tightly into a 

 socket adapted to the vernier E of a graduated circle F, fig. 2. 

 The analyser may beadjusted by turning itin its socket,and when 

 set to its proper position revolves with the vernier. The gra- 

 duated circle, with its vernier and eye-piece, is attached, for the 

 convenience of removal in packing, to a loose shelf L L, and is 

 always secured in the same position by steadying pins. The 

 upper end of the box has a circular opening of about \^ inch 

 diameter, which serves to centralize and steady the tube pass- 

 ing through it, as shown in fig. 1. S T V, fig. 1, is a three- 

 necked glass vessel to contain the liquid to be examined. W 

 is a tube graduated into inches and tenths, passing through an 

 air-tight, brass stuffing box, cemented to the vessel at V. S 

 is an opening fitted with a stopper, which is useful in filling, 

 emptying and cleansing the vessel. R is a small condensing 

 air syringe adapted to the vessel at I, and Q is a brass screw, 

 for the purpose of allowing the air to escape when requisite. 

 The method of using the apparatus is as follows: — First, 

 secure the shelf at the proper height to receive the vessel to 

 be subsequently introduced, then adjust the mirror to reflect 

 the light, of the sky or a lamp, upwards towards the analyser 

 at A, set the vernier to zero on the graduated circle, then 

 place the analysing eye-piece in the vernier, so that the ex- 

 traordinary image X is also directed towards zero; next re- 

 volve the polarizer G until X has attained its maximum of 

 obscuration, taking care not to disturb the vernier or mirror. 

 The apparatus is thus ready for experiment, and the liquid 

 to be examined must next be introduced. It may either be 

 contained within a glass vessel, S T V, such as we have 

 already described, or in a simple graduated glass tube, as 

 shown at Y, fig. 4, in which case the brass tube Z, blackened 

 on the inside, and fitted on to the lower part of the analyser, 

 is occasionally useful, and lessens the necessity of closing the 

 door to obtain a darkened chamber. The bottom of either 

 vessel should be very transparent and free from irregularity 

 or specks. These vessels are to be placed upon the shelves, as 

 shown in the drawings, and the door of the box closed. The 

 vessel S T V is contrived for the purpose of examining dif- 

 ferent depths of the same liquid without removing the vessel, 

 which is necessary when using only a simple tube, as shown 

 at Y, fig. 4. By condensing air into the vessel by means of the 

 small syringe, the liquid may be raised to any height in the 

 tube W, and it may again be lowered by allowing the air to 

 escape at the screw I. The liquid having been introduced, 

 suppose within the vessel S T V, and occupying a depth, say of 

 2 inches, the image X, which was invisible before, will then, 



