CRYSTALLOGRAPHY MINERALOGY. 309 



once properly adjusted, such a goniometer has many of the quali- 

 ties of an astronomical instrument. 



By the addition of certain accessory pieces of apparatus it may 

 be converted into an admirable instrument for determining refrac- 

 tive indices, or for measuring the angle between the optic axes of 

 biaxal crystals. 



The important additions which Mitscherlich contributed to our 

 knowledge in the facts of isomorphism, and in the angular changes 

 which crystals undergo with changes of temperature, were only 

 possible in hands possessed t>f and capable of using such a refined 

 implement as has been described. 



On the other hand, Professor W. H. Miller, adhering to the 

 very method of Wollaston, and only modifying his instrument by 

 making the disc horizontal and of dimensions enabling him to read 

 to 1 5'' and by introducing a few admirably simple contrivances 

 for supporting, centreing, and adjusting the crystal, while he uses 

 sunlight only and collimators for regulating his signals, has shown 

 by unrivalled results that the unassisted eye is competent to mea- 

 sure any crystal that has sufficiently bright faces to reflect sunlight, 

 however minute it may be, within the limits of ordinary vision. 



A most ingenious substitute for a reflective goniometer was 

 invented by Professor Miller, and may be made by any one with a 

 bit of flat board having a straight side, a couple of corks and two 

 wires or lengths of glass tubing, each bent to a right angle, at 

 about one and a half inch from the end. The form of this little 

 implement is represented in the figure. The two wires pass 

 through holes bored through a cork in perpendicular directions. 

 The tall wire is planted perpendicularly in the block of wood : of 

 the shorter wire (the bent end of which is about three-quarters of 

 an inch in length) the longer arm is passed through the large cork, 

 and, beyond, it penetrates a small cork, into which a pin carrying 

 the crystal may be stuck, or on which the crystal may be fastened 

 with cement. 



Two signals, one for direct vision and one to be seen by re- 



