146 W. ACKLAND ON A NEUTRAL TINT " SELENITE STAGE." 



or compensating film, which is rotated by the finger being pressed 

 against its projecting milled edge. 



When used, first rotate the polarizer, or analyser, until a dark 

 ground is obtained, then remove the compensating film, and place 

 the selenite stage on that of the microscope, rotate the primary film 

 until its greatest intensity of colour is obtained ; now add the com- 

 pensating film, and rotate that by the finger until the colour is 

 changed to a light neutral tint, midway between reddish purple and 



indigo blue. 



This being obtained, place the object to be examined on the 

 selenite stage and focus it. 



Now, if the thin glass covering of the object has changed the 

 tint of the background, rotate the compensating film until the 

 neutral tint is restored ; then notice the effect produced, and vary 

 slightly the position of both primary and compensating films until 

 the maximum brilliancy is obtained ; but, it may be noticed, that 

 with some objects possessing very slight depolarizing influence, the 

 maximum effect is yielded when the primary film is first rotated 

 (not as above advised until it yield the greatest brilliancy of colour), 

 but when rotated until its brilliancy is nearly at a minimum. 



Indeed, each object will require some slight variation of position 

 of one, or even of both films ; but these are easily found by trial. 



In conclusion, I boldly assert, that with this simple stage, any 

 object requiring its use can be seen to as great, if not greater, 

 advantage than with selenite stages of treble the cost, and that the 

 neutral tint I advocate will shew all of our ordinary polarizing 

 objects far more effectually than when viewed with the ordinary 

 selenite films ; but, should variation of colour be deemed necessary, 

 the mere rotation of the compensating film will at once yield an 

 abundance of tints from which to select. 



