290 On some Properties of Light. 
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Dispersive Powers or Values of =—~. 
Chromate of Jead Phosphorus....... O128 
(gr. refr.)....... 0°400 Flint glass (highest) 0°052 
Ditto, least refraction 0°262 Diamond ........ 0°038 
POA sca ssiwayes, O'F5S | Wate asses cepa nee 
Oil of cassia....... 6°139 Fluor spar.......- 0°022 
Sulphur es ieee nsee) OF130 “Cryolife oo... < on, chh eee 
_ It appears from the first of these tables, that phosphorus 
18 next to diamond in refractive power, and that the three. 
simple inflammable substances have their refractive powers 
in the order of their inflammability. Dr. Wollaston has 
placed phosphorus below horn and flint glass*; but I am 
confident that this distinguished philosopher, to whom the 
physical sciences are so deeply indebted, will find, upon 
making the experiment with prisms or lenses, that I have 
assigned the right place to that remarkable substance. The 
difference between the extreme dispersive powers in the se- 
cond table is very remarkable, and the result for oil of cassia 
indicates in that body the existence of some ingredient 
which chemical analysis has not been able to detect. 
4. On the Existence of two dispersive Powers in all doubly 
refracting Crystals. 
It has been long known, and it is indeed obvious, from a 
simple inspection of the images formed by a prism of Ice- 
land crystal, that the one image is more coloured than the 
other, or that the actual dispersion of the one refraction is 
greater than the dispersion of the other, in the same manner 
as the dispersion of a prism of flint glass with a refracting 
angle of 12 degrees, is greater than the dispersion of a 
prism of the same glass with an angle of only 10 degrees. 
Dr. Wollaston, who was the first person that examined 
the subject of dispersive powers with philosophical accu- 
racy, makes the dispersive power of Iceland spar considere 
ably above water, and even above diamond. Upon repeat- 
ing this experiment, with the least refracted image, I found 
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the dispersive power, or the value of = —-, to be 0:026, very 
considerably below water, which stands at 0°035 of the 
scale; and [ therefore concluded that Dr. Wollaston had 
examined the greatest refraction, while I had examined the 
least, and that the vast discrepancy between our measures 
* Dr. Wollaston is satisfied that his original estimate was erroneous, and 
that Dr. Brewster’s determination is very near the truth, H, D. 
arose 
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