by doubly refracting crystals. 
13 
If we now take a rhomboid of certain specimens of yellow 
calcareous spar, and perform with it the experiments which 
have just been described, we shall obtain a series of en- 
tirely different results. The two images will now be found 
to differ both in colour and intensity, the extraordinary 
image having an orange yellow hue, while the colour of 
the ordinary image is a yellowish white. This difference 
of colour is distinctly related to the axis of the crystal, 
and increases with the inclination of the refracted ray to the 
short diagonal of the rhomb. It is a maximum in the 
equator, while along the axis the two images have exactly 
the same colour and intensity. In every position, however, 
the combined tints of the two images are exactly the same 
as the natural tint of the mineral. In comparing the inten- 
sities of the two images, the extraordinary one appears 
always the faintest, so that there is an interchange of rays ; 
and while the extraordinary force carries off several of the 
yellow rays from the ordinary image O, the ordinary force 
at the same time takes to itself several of the white rays from 
the extraordinary image E ; for if this were not the case, the 
extraordinary image would always have the greatest intensity, 
whereas, in consequence of its exchanging yellow for white 
light, it becomes actually fainter than the ordinary image. 
If we call m and n the maximum number of rays which 
the extraordinary and the ordinary image interchange, and 
(cp) the inclination of the refracted ray to the axis, the intensi- 
ties may be represented by the following formulas when the 
crystal is exposed to common light. O = \ Q + sin . 2 (pm — 
sin 2 <p n and E = \ Q -f- sin . 2 <pn — sin . 2 <p m. The values of 
m and n vary in different crystals : they are always of different 
