598 J. W. EVANS ON THE DETERMINATION OF MINERALS UNDER 



ment is employed. At the same time tlie complication of the 

 phenomena caused by the rotation of the object renders a 

 systematic procedure, such as I shall describe, very desirable, if 

 mistakes are to be avoided. 



Centring. In such an instrument an arrangement for centring, 

 by which the axis of the microscope can be adjusted so that it 

 may pass through the centre of the stage, is absolutely necessary. 

 The centring may be carried out by placing a rock-slice in focus 

 under the microscope, noticing the point round which the object 

 seems to rotate, and bringing this to the centre of the field by 

 means of the centring screws. 



Nose-fiece. The mechanism for centring should be applied to 

 the nose-piece and not to the stage, since it is the former which 

 is most liable to be displaced, especially if a double or triple 

 nose-piece for interchanging objectives be employed. The use 

 of a clutch, first employed, I believe, by Nachet, by which 

 objectives can be rapidly attached or removed, is preferable. 

 Kecently a lateral sliding arrangement has been introduced, but 

 I do not think that it possesses any points of superiority over 

 the clutch. 



Movements of One Nicol. One or both nicols should be 

 capable of separate rotation, and one at least should be capable 

 of being rapidly thrown out of the course of the light so that 

 the observation may be made with one nicol only. The nicol 

 that remains in position should be so placed that it allows light 

 vibrating right and left to pass, for with the usual disposition 

 of the mirror the light reflected from it is polarised so that 

 more of it already vibrates in this than in other directions. 

 There is consequently an appreciable saving of light with this 

 position of the nicol.* 



It is usual to remove the upper nicol or analyser, but F. E. 

 Wright recommends the removal of the lower nicol or polariser. 

 This has the advantage that the field is not affected in focus 

 or position, when the nicol is moved in or taken out. 



* To ascertain in what direction light traversing a nicol vibrates, 

 the nicol should be inserted alone and a rock-slice containing biotite 

 flakes showing strong pleochroism placed on the stage and rotated 

 till a flake is in the position of maximum darkness. The direction 

 of the cleavage of this flake will then be parallel to that of the vibration 

 of the nicol. 



