This sphere of refractive material he calls the 'capitellum' of the chordotonal thread.
It is perfectly transparent, yet it has refractive qualities.
Water has a refractive Power in a middle degree between those two sorts of Substances, and probably is of a middle nature.
This effect, like the refractive power and focal distance, varies in degree in different diaphanous substances.
The refractive and dispersive powers are not high, so that the cut stones display little brilliancy or “fire.”
It must evidently diminish as the refractive index increases.
The variation of refractive index with density has been the subject of much experimental and theoretical inquiry.
The amount of refractive error is very great if an object 23lies near the horizon.
Where the surface was hard and dense, Brewster found the refractive index to approach that of semi opal.
In this case the earth is the kiln, and the stars the object that is viewed through the refractive medium.