NOTES AND MEMORANDA. 



193 



and the slit opened or sliut. One of tbe grooves Las a graduated 

 scale, by which the length of the oi^en slit is shown. 



The apparatus is fixed on the Microscope in such a way that the slit 

 in the screen is illuminated by the light of the window. If the glass c 



Ma^ 



TTtrmWt 



Dtw 





is then raised to an angle of 45° with the horizon, no difficulty will be 

 experienced in seeing a reflected image of the slit li on the glass, in 

 the middle of the diaphragm of A. Hence, looking through the glass 

 into the Microscope, we see simultaneously the interference image and 

 the reflected image of the slit, which appears as a brilliant line. It 

 now remains to bring one of the two extremities of the image of th^ 

 slit over one of the visible poles of the optic axes. That this may be 

 done, the screen B is movable on the plate m, which serves as its 

 base, and with which it is fixed by the screw / on the rod C ; the two 

 screws </ are inserted in the oblong openings /, and by loosening these 

 screws a little the screen can be moved upon the base-plate to the 

 right or to the left, so that the desired position may be obtained. By 

 opening the slit we can make the other end of its image coincide with 

 the pole of the other optic axis in the interference image. This 

 position is represented in Fig. 1. 



The scale shows the length of the opening of the slit, and the 

 distance of the poles of the optic axes. Measuring by the same 

 method the distance of the poles for mica, of which the apparent angle 

 of the optic axes is known, the proportion is found of the two distances, 

 which enables us to calculate easily the angle of the optic axes. The 

 scale is carefully graduated in fifths of a millimetre, and with the help 

 of a lens the third of a degree can easily be distinguished, and the 

 error in the results is found not to exceed one or two degrees. 



It will be understood that this method is only applicable to 

 minerals in which the angle of the optic axes is not large, or the 

 poles of the axes would be no longer visible. But it is easy to put 

 the preparation in a small cup of oil, and then measure the angle 

 in the oil. 



Artificial Crystals of Gold. — A few years ago some objects from 

 America were exhibited which, under a power of 150 to 200, looked 

 like microscopic fern-leaves gilt (see woodcut), but which were stated 

 to be crystals of gold. The process by which they were produced was 



VOL. II. o 



