192 



NOTES AND MEMORANDA. 



that we have two thin plates of the mineral to be examined, cut 

 perpendicularly to one another. In the case of all minerals whose 

 cleavage in one direction is very perfect, it is difficult, and often even 

 impossible, to cut a thin plate normal to the direction of the cleavage. 

 It was, therefore, desirable to be able to determine the apparent angle 

 of the optic axes by direct measurement with the Microscope. In all 

 cases where a mineral only becomes transparent when the plates are 

 very thin, the determination of the angle of the optic axes can only be 

 eifected in general with the Microscope. 



To arrive at this result, the distance of the poles of the optic axes 

 of a mineral, as seen in the interference image, must be compared with 

 this same distance in a film of biaxial mica, for which the angle of the 

 optic axes has been determined by an instrument specially designed to 

 measure it. 



To make a sufficiently exact comparison of the distances of the 

 poles in the film of mica, and in the thin plate of the mineral to 

 be examined, we must be able to measure exactly these two distances 

 in the Microscope. As the eye-piece is removed in order to see the 

 interference image, the eye-piece micrometer cannot be used without 

 employing lenses by which the interference image is distorted. 



The following form of apparatus has accordingly been designed by 

 Professor de Lasaulx. On the edge of the setting of the upper Nicol, 

 a brass cover A (Fig. 1) is fixed by screws d, having a diaphragm of 



Mj./ 



h B 



the same size as the glass which covers the Nicol. At one side there 

 is a horizontal axis which can be turned by the screw a, and at the 

 same time this axis and the plate which it supports can be turned round a 

 screw h. This axis holds an ordinary covering glass c, through which 

 may be seen the image in the Microscope. A rod C carries the other 

 part of the apparatus B, which consists of a blackened rectangular 

 screen with a very fine horizontal slit h ( Fig. 2) in the middle. On 

 one side the screen has a small slide /, which by the screw e passing 

 through I, may be moved in the grooves h to the right or to the left, 



