< LIFE OF SIR ISAAC NEWTON. tion to make the colours in the three fringes above described ? Also, whether the rays, in passing by the edges and sides ol bodies, are not bent several times backwards and forwards with an eel-like motion — the three fringes arising from three such bendings ? His inquiries on this subject were here interrupted and fiever renewed. His Theory of the COLOURS of NATURAL BODIES was commu nicated to the Royal Society, in February, 1675. This is justly regarded as one of the profoundest of his speculations. The fun damental principles of the Theory in brief, are : — That bodies possessing the greatest refractive powers reflect the greatest quantity of light ; and that, at the confines of equally refracting media, there is no reflection. That the minutest particles of al most all natural bodies are in some degree transparent. That between the particles of bodies there are pores, or spaces, either empty or filled with media of a less density than the particles themselves. That these particles, and pores or spaces, have some definite size. Hence he deduced the Transparency, Opacity, and colours of natural bodies. Transparency arises from the particles and their pores being too small to cause reflection at their com mon surfaces — the light all passing through ; Opacity from the opposite cause of the particles and their pores being sufficiently large to reflect the light which is " stopped or stifled7' by the multitude of reflections ; and colours from the particles, accord ing to their several sizes, reflecting rays of one colour and trans mitting those of another — or in other words, the colour that meets the eye is the colour reflected, while all the other rays are transmitted or absorbed. Analogous in origin to the colours of natural bodies, he con sidered the COLOURS OF THIN PLATES. This subject was interest ing and important, and had attracted considerable investigation. He, however, was the first to determine the law of the produc tion of these colours, arid, during the same year made known the results of his researches herein to the Royal Society. His mode of procedure in these experiments was simple and curious. He placed a double convex lens of a large known radius of curvature, the flat surface of a plano-convex object glass. Thus, from