Before concluding this chapter, there is one topic peculiarly deserving our notice, namely, the change of colour produced in bodies by continued exposure to light. The general effect of light is to diminish or dilute the colours of bodies, and in many cases to deprive them entirely of their colour. Now, it is not easy to understand how repeated undulations propagated through a body could diminish the size of its particles, or how the same effect could be produced by a multitude of reflections from particle to particle. But if light is attracted by the particles of bodies, and combines with them, it is easy to conceive that, when the molecules of a body have combined with a great number of particles of a green colour, for example, their power of combination with others will be diminished, and, consequently, the number of particles of any colour absorbed or detained must diminish with the time that the body has been exposed to light; that is, these particles must enter into the transmitted and reflected pencils, and diminish the intensity of their colour. If the body, for example, absorbs red light, and transmits and reflects green, then if the quantity of absorbed red light is diminished, it will enter into the reflected and transmitted pencils, and, forming white light by its mixture with a portion of the green rays, will actually dilute them in the same manner as if a portion of white light had been added.[28] CHAPTER VIII. _Newton’s Discoveries respecting the Inflection or Diffraction of Light—Previous Discoveries of Grimaldi and Dr. Hooke—Labours of succeeding Philosophers—Law of Interference of Dr. Young—Fresnel’s Discoveries—New Theory of Inflection on the Hypothesis of the Materiality of Light._ Although the discoveries of Newton respecting the _Inflection of Light_ were first published in his _Optics_ in 1704, yet there is reason to think that they were made at a much earlier period. Sir Isaac, indeed, informs us, in his preface to that great work, that the third book, which contains these discoveries, “was put together out of scattered papers;” and he adds at the end of his observations, that “he designed to repeat most of them with more care and exactness, and to make some new ones for determining the manner how the rays of light are bent in their passage by bodies, for making the fringes of colours with the dark lines between them. But I was then interrupted, and cannot now think of taking these things into consideration.” On the 18th March, 1674, Dr. Hooke had read a valuable memoir on the phenomena of diffraction; and, as Sir Isaac makes no allusion whatever to this work, it is the more probable that his “scattered papers” had been written previous to the communication of Dr. Hooke’s experiments. The phenomena of the inflection of light were first discovered by Francis Maria Grimaldi, a learned Jesuit, who has described them in a posthumous work published in 1665, two years after his death.[29]