THE SPECTEUM-MICKOSCOPE. 
77 
satisfactory, and the dispersion as great or greater than usually 
required, and to see them well the two analyzing prisms should 
be combined. A very weak solution of blood is a good mode- 
rate test, and requires the medium amount of dispersion which 
is obtained by the analyzer containing two rectangular flint- 
glass prisms. As a good test for what cannot be well seen 
except with a small dispersion, I may mention the double 
oxalate of chromium and soda, which is also a very interesting 
object in other respects. The best way to prepare it is to 
place a few drops of a strong hot solution on glass, so that 
a number of minute crystals may be deposited on cooling, 
and then to leave the rest to deposit by spontaneous evapora- 
tion. Very well fm*med microscopic crystals are then ob- 
tained, which, according to the position in which they lie, or 
their thickness, are a beautiful blue or purple, or various 
shades of green and red. I know no object which shows in t a 
more striking manner the effects of dichroism in both senses 
of the term. It is, indeed, polychroic, and is an excellent 
object to exhibit with an arrangement I am now carrying out, 
so as to be able to make use of the microscope as a dichroi- 
scope. 
When the crystals are moderately thin, they show a broad 
absorption band in the yellow and yellowish-green, and 
transmit the dark green, with a variable amount of red and 
blue, according to the position of the crystal ; but when 
thicker, the absorption band becomes darker at each end, so 
that we appear to have two absorption bands, one in the red 
and another in the dark green, separated by an obscure 
division not well seen except the dispersion be small. 
It must not be thought that the subjects I have chosen for 
illustration by any means exhaust those to which the instru- 
ment may be applied. Whenever colour is a character of any 
importance, then to a still greater degree is its more accurate 
study by means of the prism likely to yield valuable facts. 
