THE INTERFERENCE OF LIGHT. 
215 
On using such an arrangement for the cases just mentioned, I accordingly found 
bands produced. 
(8.) In any case to see the bands well, especially towards the violet end of the 
spectrum, a strong light is requisite : that of the sun direct, shaded by a blue glass, 
or at least that of the bright part of the sky near the sun, was usually thrown in by 
the shutter apparatus ; and though a small telescope was usually employed, yet 
the bands, in cases where they are vivid and broad, may be seen by the naked 
eye ; and for such cases lamp-light may be used, but it will not suffice for the more 
delicate. 
(9.) When crystallized substances are employed as plates other peculiar phenomena 
are presented. 
A plate of calc-spar (formed by the natural cleavage) with oil of cassia gives 
two distinct sets of bands ; the one finer and narrower than the other, which 
about the middle of the spectrum may be seen distinctly superimposed one on the 
other. 
On applying a Nicol-prism each set disappears alternately, leaving the other visible 
at each quarter of a revolution of the analyser ; showing them due to the two oppo- 
sitely polarized pencils. It is easily ascertained that the Jiner bands belong to the 
extraordinary, the broader to the ordinary ray. 
(10.) A plate of quartz, cut perpendicular to the axis, with oil of sassafras, gives 
very distinct bands, which may be seen to be in fact composed of two sets superim- 
posed and nearly coinciding, since when the plate is inclined (as in position 1, see 
fig. 2) at intervals throughout the spectrum the near coincidence of the dark spaces 
of one set, with the bright of the other, occasions an extreme faintness in the bands. 
In this case the two pencils deviate sufficiently from the axis to approximate to plane 
polarized light, and thus the Nicol-prism causes the indistinctness to disappear at 
each quarter of a revolution by stopping one of them alternately. 
(11.) For an explanation of the general phenomena of the formation of bands under 
the conditions specified, the simple interference-theory suffices. 
Of the homogeneous pencil going to form any one ray of the spectrum, -that half 
which passes through the thicker part of the prism is more retarded than that 
through the thinner ; but uniformly, and in proportion to the difference of refraction, 
throughout the spectrum. 
The plate of glass, however, having different indices for the several primary rays 
from those of the medium, interrupts this uniformity, and causes the one part of the 
pencil to be always retarded in an increasing ratio with respect to the other, through- 
out the spectrum ; and as this difference of retardation amounts successively to an 
odd or an even multiple of a half wave-length, the rays will be in discordance or 
accordance, or give a dark or bright band accordingly. 
(12.) But to account for the different conditions which determine the number and 
