means of imperfectly transparent, yet light-transmitting mediums; and in the second such as are exhibited when the medium is in the highest degree transparent.
X.
DIOPTRICAL COLOURS OF THE FIRST CLASS.
145.
Space, if we assume it to be empty, would have the quality of absolute transparency to our vision. If this space is filled so that the eye cannot perceive that it is so, there exists a more or less material transparent medium, which may be of the nature of air and gas, may be fluid or even solid.
146.
The pure and light-transmitting semi-transparent medium is only an accumulated form of the transparent medium. It may therefore be presented to us in three modes.
147.
The extreme degree of this accumulation is white; the simplest, brightest, first, opaque occupation of space.
148.