colours which accompany them in calcareous spar. 275 
menon, and is the only perfect example in which the comple- 
mentary fringes are seen at the same instant.* In the spe- 
cimen shown in Fig. 7, one of the interrupting planes gives 
remarkably minute fringes, while the other forms them of a 
larger size.-f 
Sect. II. On the position and character of the interrupting plane. 
In every specimen of calcareous spar which possesses the 
property of multiplying and colouring the images, there is a 
plane ABCD, Fig. 1. stretching across the crystal. This plane, 
which I shall call the interrupting stratum , has not the most re- 
mote likeness to a fissure or fracture, but resembles rather a 
thin vein or film cohering to the two prisms between which it 
is interposed. The lines AB, CD which form the termination 
of the stratum, are distinctly marked on the natural faces of 
the crystal, and form straight lines perpendicular to the 
shorter diagonal FE ; and the rhomboid is divided bv the in- 
terrupting stratum into two equal prisms ABCDGE, ABCDHF, 
having the angles ABE, BAF each equal to 39 0 . 
If the plane ABCD is a fissure or a stratum of air, as has 
been supposed, it is demonstrable that a ray of light incident 
at an angle of 37 0 upon AB will suffer total reflection, and 
therefore no light will be transmitted through the rhomboid. 
So far, however, from this being the case, there is actually no 
angle of incidence at which total reflection takes place at the 
second surface AB, and consequently there is no physical 
* An imperfect example of this I have given in the Phil. Trans. 1814, p. 227. 
Plate VIII. fig. 3. 
f In the specimens represented in Plate XI. fig. 8 and 1 1, of my Treatise on New 
Philosophical Instruments, the fringes are very large. 
N n 2 
