refractive and dispersive Powers . 373 
So high is the dispersive power of this oil, that, in refractions 
from flint glass into it, the red rays are refracted more than the 
violet. 
It must be observed that, in this experiment, when the angle 
of reflection within a triangular prism exceeds 6 o°, the angle of 
emergence is such as would alone occasion the red rays to 
appear lowermost; but, when the glass used is rectangular, the 
refraction at emergence has an opposite effect; any reversion 
of colour will therefore be in some degree corrected, and may not 
be seen, unless the dispersive power of the medium in contact 
much exceeds that of the glass. 
A case of refraction with an inverted order of colours, has 
been observed by Dr. Blair,' * in a compound object-glass, 
where crown-glass was in contact with oil of turpentine. From 
trials with lenses, he likewise inferred, that several other fluids 
have the same effect, when applied to that glass. 
With this glass, and also with plate-glass, I have tried oil of 
turpentine, and many other fluids that afford a similar reversion 
of colours, as linseed-oil, olive-oil, the essential oils of berga- 
mot, lemon, lavender, pennyroyal, and peppermint, strong nitric 
acid, and many artificial compounds that I shall presently have 
occasion to mention. 
The dispersive power of fluor spar is the least of any sub- 
stance yet examined ; so that, although its refractive power is 
also remarkably low, (considering its great specific gravity,) a 
prism of fluor, in contact with water or alcohol, shows the 
prismatic colours to be refracted in an inverted order. 
With heavy spar, the instances of reversion are very nume- 
rous, as its dispersive power is low, and is accompanied with 
* Edinb, Trans. Vol. III. 
sc 
MDCCCII, 
