Theory of Colours — trans. Charles Lock Eastlake (1840)

Light under these circumstances may be affected by three conditions. First, when it flashes back from the surface of a medium; in considering which _catoptrical_ experiments invite our attention. Secondly, when it passes by the edge of a medium: the phenomena thus produced were formerly called _perioptical_; we prefer the term _paroptical_. Thirdly, when it passes through either a merely light-transmitting or an actually transparent body; thus constituting a class of appearances on which _dioptrical_ experiments are founded. We have called a fourth class of physical colours _epoptical_, as the phenomena exhibit themselves on the colourless surface of bodies under various conditions, without previous or actual dye (βαφή).--Note K.

141.

In examining these categories with reference to our three leading divisions, according to which we consider the phenomena of colours in a physiological, physical, or chemical view, we find that the catoptrical colours are closely connected with the physiological; the paroptical are already somewhat more distinct and independent; the dioptrical exhibit themselves as entirely and strictly physical, and as having a decidedly objective character; the epoptical, although still only apparent, may be considered as the transition to the chemical colours.

142.

If we were desirous of prosecuting our investigation strictly in the order of nature, we ought to proceed according to the classification which has just been made; but in didactic treatises it is not of so much consequence to connect as to duly distinguish the various divisions of a subject, in order that at last, when every single class and case has been presented to the mind, the whole may be embraced in one comprehensive view. We therefore turn our attention forthwith to the dioptrical class, in order at once to give the reader the full impression of the physical colours, and to exhibit their characteristics the more strikingly.

IX.

DIOPTRICAL COLOURS.

143.

Colours are called dioptrical when a colourless medium is necessary to produce them; the medium must be such that light and darkness can act through it either on the eye or on opposite surfaces. It is thus required that the medium should be transparent, or at least capable, to a certain degree, of transmitting light.

144.

According to these conditions we divide the dioptrical phenomena into two classes, placing in the first those which are produced by 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.

Transparency itself, empirically considered, is already the first degree of the opposite state. The intermediate degrees from this point to opaque white are infinite.

149.

At whatever point short of opacity we arrest the thickening medium, it exhibits simple and remarkable phenomena when placed in relation with light and darkness.

150.

The highest degree of light, such as that of the sun, of phosphorus burning in oxygen, is dazzling and colourless: so the light of the fixed stars is for the most part colourless. This light, however, seen through a medium but very slightly thickened, appears to us yellow. If the density of such a medium be increased, or if its volume become greater, we shall see the light gradually assume a yellow-red hue, which at last deepens to a ruby-colour.--Note L.

151.

If on the other hand darkness is seen through a semi-transparent medium, which is itself illumined by a light striking on it, a blue colour appears: this becomes lighter and paler as the density of the medium is increased, but on the contrary appears darker and deeper the more transparent the medium becomes: in the least degree of dimness short of absolute transparence, always supposing a perfectly colourless medium, this deep blue approaches the most beautiful violet.

152.