Dictionarium polygraphicum. Of the colours of thin plates.

Dictionarium Polygraphicum:
Or, The Whole Body of Arts Regularly Digested.
Vol I.
London: Printed for C. Hitch and C. Davis in Pater-noster Row, and S. Austen in St. Paul's Church Yard. MDCCXXXV.
1735
As says of different colours are separated by the refraction of prisms, and other thick bodies; so are they separated, though in a different manner in the thin lamellæ or plates of any pellucid matter, v. g. the bubbles rais'd in water thickned by soap, &c.

For all lamellæ under a determinate thickness, transmit rays of all Colours, without reflecting any at all, but as they increase in thickness, in arithmetical proportion, they begin to reflect first blue rays; then in order green, yellow and red, more and more mixt and diluted, till at length, arriving at a certain thickness, they reflect rays of all Colours, perfectly intermixt with white.

But in whatsoever part a slender lamellæ reflects any one colour, v. g. blue, in that part it always transmits the opposite colours, v. g. red or yellow.

It is found by experiment, that the difference of Colour of a plate, does not depend upon the medium that encompasses it, but the degree of vividness does; cæteris paribus, the Colour will be more vivid, if the denser medium be encompass'd with a rarer.

A plate, cæteris paribus, reflects more light, as it is thinner, as far as a certain degree of thinness; beyond which, it reflects no light at all.

In plates, the thickness of which increase in an arithmetical progression of the natural numbers, 1, 2, 3, 4, 5, &c. If the first or thinnest reflect any homogeneal ray, the second will trans mit it, and thus is the same ray alternately reflected and trans mitted, i. e. the plates corresponding to the odd numbers, 1, 2, 3, 5, 7, &c. will reflect the same rays, that those corresponding to the even ones, 2, 4, 6, 8, &c. transmit.

Hence an homogeneous Colour in a plate is said to be of the first order; if the plate reflect all the rays of that Colour. In a plate, whose thinness is in a threefold proportion to the first, it is said to be of the second order, in another, whose thinness is five times that of the first, it is said to be of the third order, &c.

A Colour of the first order, is the most vivid of any, and successively the vividness of the Colour increases, as the quantity of the order increases; the more the thickness of the plate is increased, the more Colours it reflects; and those of more different orders.

In some plates the Colour will vary as the position of the eye varies, in others it is permanent.