Measuring refraction under the Compound Microscope. 111 
stant error. By this mode the following approximate results were 
obtained for fluor spar, alum and crown glass: 
Fluor spar, - - 1:419 1-434 
Alum - - 1-445 1-457 
1-4 
a ¢ 
Crown sla optical tables. 
If the microscope is furnished with a pair of Nicols’ prisms or 
tourmalines, but little management is required to arrange its parts 
so that it will answer asa polariscope for large objects, and for 
viewing the rings around the axes of crystals: it also furnishes 
a good extemporaneous apparatus for examining the circular po- 
larization of liquids: the necessary arrangements and additions 
‘ will readily suggest themselves. Also by means of the grad- 
nated circle the inclinations of the axes of biaxial crystals can 
e measured, 
The inclination of the ordinary to the extraordinary ray when 
_ and thus at certain inclinations to the optical axis, can with the 
microscope be readily determined even for very small crystals. 
or this purpose a piece of tin-foil is firmly pasted on a glass 
plate and afterwards with the point of a knife a fine slit is made 
across it: the crystal is laid over this slit and viewed by perpen- 
dicularly srananittted light and a power of from 50 to 200 diam- 
: eters, and the distahce apart of the two images of the slit, or the 
7 tangent of the aifgt of their inclination measured with the eye- 
piece micrometer, "The crystal is then turned on its edge and its 
thickness measured in the same way. If ¢ = the thickness, and 
t 
of from 50 
tion as ij; ofa degree in be measured, 
Munich, Oct. 1st, 1855. 
the light falls perpendicularly on two parallel faces of the crystal 
e 
d the distance of the:two rays apart, we have —-= tang. of an- — 
