230 W, HISLOP ON A NEW ANALYZING SELENITE STAGE. 



are mounted in thin brass cells of different sizes to fit the 

 three cells, each selenite to its own cell, the lower one being the 

 smallest ; a pin or stop being so arranged that the selenite will 

 only drop into one fixed position with regard to the cell. By 

 means of the toothed wheels the selenites are then arranged with 

 their axes of polarization parallel to each other, at the same time 

 that the figure 1 on the discs appears outwards. The upper plate 

 is then screwed on and the adjustments are complete. The selen- 

 ites not being fixed in the revolving cells, are easily removed for 

 cleaning, and as easily replaced, and when the discs are all arranged 

 to show the figure 1, and the polarizer and analyzer placed in 

 the proper position, we have the nearest possible approach to white 

 light. In order to effect this more readily both polarizer and 

 analyzer should be marked. 



A lever, having a small toothed w£eel attached to it, is placed at 

 the top, by moving which all the wheels are placed in connexion, 

 and the whole system may be revolved by turning either of the 

 discs. 



By registering the position of the graduated discs by the aid of 

 the engraved numbers, any required effect may be reproduced by 

 simply placing the discs in the same relative positions. This can 

 be done by simple inspection, without removing the object or the 

 selenites. In fact, a table may be prepared, indicating the different 

 tints to be produced by any particular position of the numbered 

 discs. 



The instrument can be used with any microscope, requiring no 

 especial fittings, and could also be used with a telescope, for analyz- 

 ing the light of the heavenly bodies. 



I must confess that the results obtained even exceed my antici- 

 pations. In addition to much more being brought out in conse- 

 quence of the adaptation of the depolarizing power to the object, 

 there is also an amount of exquisite definition at command which it 

 is exceedingly gratifying to the observer, and which is also sugges- 

 tive of some points in illumination, to which I may be able to 

 advert in a future paper. 



I have named the instrument the Analyzing Selenite Stage, in 

 consequence of its possessing the property of optical analysis ; 

 and in another form of it I have an arrangement whereby either 

 one, two, or three films may be withdrawn from the field with- 

 out interfering with the mechanical action. 



