Measuring refraction under the Compound Microscope. 109 
instrument. To measure the angle the condenser is removed 
and the crystal is to be turned on the axis of the instrament 
‘ till one of the faces is seen by the fixed lens brightly illumin- 
ated: it is then to-be turned till the bright reflexion is just 
: ne on the vernier is then made to coincide with oO 
at the moment of the disappearance of the light through the same 
portion of the lens, as for example the upper, the error will not 
be greater than 5’ or 6’. It is evident that by this method ap- 
" proximate measurements can be obtained on crystals whose faces 
. are large enough ¢o be seen, when brightly illuminated, by an 
the focus of the eye-lens, and the greater lengthening and short- 
ening of the projection of the edge of intersection furnishes a 
4 means of knowing when the adjustment is effected. 
S| When the crystals are exceedingly small and numerous, the 
1 point of the wax is to be dipped into them, some particular crys- 
. tal selected under the microscope and the others dissected away 
With the aid of a fine needle. It is plain that these three modes 
of measuring microscopic crystals are adapted to the ordinary re- 
flective goniometer, 
eter be taken and to one end of it be attached the posterior lens” 
of the inch obj 
elescope an 
i the com 
ex of refraction.—If a brass tube of about 3-inch in diam- _ 
Pgs 
i 
