90 DOVE’S DESCRIPTION OF AN APPARATUS 
manner the plates of different crystallized bodies are combined, in order 
to examine the positive or negative character of their axes. 
If, instead of white, either homogeneous or dichromatic light is to 
be niade incidént, small rings ef wood one inch in diameter, with 
coloured glasses, must he fastened before the aperture of the polarizing 
prism e. When the concentrated light of a lamp giving white light falls 
upon dichromatic glasses, they exhibit with biaxal crystals different opti- 
cal axes for the various colours, and with uniaxal crystals they yield 
beautiful changes of differently coloured rings. Blue glasses, which sepa- 
rately tratisniitted the extremes of the spectrum, exhibit (in arragonite, 
for instance,) the inner curve divided into two particoloured vacant 
spaces and corresponding changes within each ring; on the contrary; 
the two inner rings in the Iceland spar are exhibited of a deep red sur- 
rouiided by violet rings gradually passing more and more into each 
other, during which, lighted by a flame of spirits of wine coloured 
by chloride of strontium, the three inner rings are violet, to which three 
réd Ones then succeed, and so forth. Through a ruby glass we now obtain 
only a very homogeneous red, then dark rings, ih the red field of view. 
A flame of spirits of wine coloured yellow with common salt, or nitrate 
of soda, yields the most beautiful phenomenon. The dark rings and the 
junction-curves of the different systems of rings of twin-crystals of ar- 
ragonite then appear in the linear and circular light with the utmost 
distinctness. For blue and violet it is best to employ the colours of the 
spectrum. The condensing-lens is then removed, in order that the light 
may fall directly upon the aperture of the polarizing prism. 
The apparatus shown in Plate II. fig. 3. serves to analyse the light 
by reflexion, and is screwed into the pillars, instead of the analysing 
prism. The screw at « holds a concave lens of an equal focal distance. 
The unbordered mirror inclined at the angle of polarization is 4 inch 
long and § inch wide. ~A line is drawn over the three parts of the hinge 
q on the left side of fig.1. Ifthe parts of this line form one straight line, 
the rod dc is inclined towards a horizontal mirror at tlie angle of pola- 
rization. If & and v are placed aside, the light polarized by reflexion 
may be analysed either linearly by the prism, by the mirror in w, or circu- 
larly by means of f. But in order to examine larger cooled glasses 
in circularly polarized incident light, I employ a larger lamina of mica 
than that in g, which may be called g, and which fixed to the screw of — 
the condensing-lens p is screwed directly upon a wooden ring of 2 
inches internal diameter. The axis of this mica lies like that of the 
thin plate in g, which is turned aside. The concave lens in w is 
taken out, and the stand supporting the cooled glasses is brought to 
the distance most suitable to the eye. By holding the glasses in the 
hand, the various phenomena of the linear and circular light may be 
observed without alteration of the apparatus. If the glass be held 
