90 DOVk's DESCKXPTION Ol- 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, eitlier homogeneous or didhromatic light is to 

 be made incident, small rings of wood one inch in diameter, with 

 coloured glasses, must be 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 trailsnlitted 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 lings in the Iceland spar are exhibited of a deep red sur- 

 rounded 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 

 red ones then succeed, and so forth. Through a ruby glass we now obtain 

 only a very homogeneous red, then dark rings, in the red field of view. 

 A flame of spirits of wine coloured yellow with common salt, or nitrate 

 of soda, yields the most beautiful phsenomenon. The dark rijigs 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 pillar s^ instead of the analysing 

 prism. The screw at u holds a concave lens of an equal focal distance. 

 The unbordered mirror inclined at the angle of polarization is ^ inch 

 long and 4 inch wide. A line is drawn over the three parts of the hinge 

 y on the left side of fig. 1. If the parts of this line form one straight line, 

 the rod 6 c is inclined towards a horizontal mirror at the angle of pola- 

 rization. If k and V are placed aside, the light polarized by reflexion 

 jnay be analysed either linearly by the prism, by the mirror in w, or circu- 

 laily by means of/. 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 u 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 phaenomena of the linear and circular light may be 

 observed M'ithout alteration of the apparatus. If the glass be held ^ 



