THE MICROSCOPE BY MEANS OF THEIR OPTICAL CHARACTERS. 617 



Iii that case to make the object-image coincide with the dia- 

 phragm, a lens, or a combination of a lens with the eye-piece, is 

 focused on the position of the diaphragm and the tube raised till 

 the object is seen in focus. The angle of the cone illuminated by 

 the condenser is, however, diminished by the elevation of the 

 objective. 



I now proceed to describe some of the phenomena seen in the 

 directions-image, especially those which may be easily observed 

 in minerals in thin sections and afford important information with 

 regard to their optical characters, as well as the directions in 

 which they have been cut. 



Interference Colours in the Directions-Image. When the 

 directions-image is examined between crossed nicols it shows 

 in the centre of the field the same interference colour as that 

 seen in the object-image. From the centre outwards this passes 

 into other colours corresponding to different amounts of relative 

 retardation which may be greater or less than that in the centre. 

 The colours move with the stage as it rotates without sufferino- 

 any change of configuration. 



Isogyres. At the same time the field is traversed by dark 

 bands or brushes, which constitute the isogyre* As the rotation 

 proceeds, this, as a rule, changes both its position and its shape 

 and from time to time leaves the field altogether. 



When the stage is in the position corresponding to extinction 

 in the object-image, in other words when the vibrations in the 

 plane of the crystal section are parallel to the cross wires, the 

 isogyre passes through the centre of the field and is known as a 

 central isogyre (figs. 7, 8, 13, 16-21). 



The visible portion of the isogyre consists in the majority of 

 cases of a single dark band, which usually expands towards the 

 margin of the field to form a less definite brush. This moves 

 four times across the field as the stage rotates, being usually 

 lost to view in the intervals. 



In other cases the isogyre consists of two dark bands which 

 either meet in a cross or form the two branches of an hyperbola. 



The following special types of central isogyres formed of a 

 single band may be distinguished. 



A symmetrical isogyre is straight and parallel to one of the 



* F. Becke, Min. Petr. Mitt. (Tschermak), vol. xxiv., 1905, pp. 1-34, and 

 Min. Mag. vol. adv.-, 1907, pp. 27G-80, and J. W. Evans, ib. pp. 230-3. 



