maladies of the eye, the patient, it seems, suffers the constant presence of light without external light. The exciting principle in this case is thus proved to be within, and the conclusion of the physiologists is that external light is only one of the causes which produce luminous and coloured impressions. That this view was anticipated by Newton may be gathered from the concluding "query" in the third book of his Optics.
NOTE K.—Par. 140.
"Catoptrical colours. The colours included under this head are principally those of fibres and grooved surfaces; they can be produced artificially by cutting parallel grooves on a surface of metal from 2000 to 10,000 in the inch. See 'Brewster's Optics,' p. 120. The colours called by Goethe paroptical, correspond with those produced by the diffraction or inflection of light in the received theory.—See Brewster, p. 95. The phenomena included under the title 'Epoptical Colours,' are generally known as the colours of thin plates. They vary with the thickness of the film, and the colour seen by reflection always differs from that seen by transmission. The laws of these phenomena have been thoroughly investigated. See Nobili, and Brewster, p. 100."—S. F.