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culiarly deserving of 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 colour of bodies, and in many cases to deprive them entirely of their colour. Now, it is not easy to understand now 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 reflexions 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 dimmish 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.*

* In a note here appended, Sir D. Brewster remarks that " since the two preceding chapters were written, I have had occasion to confirm and extend the views which they contain by many new experiments." And ho refers in the larger edition to the interesting discoveries of Professor Stokes of Cambridge, relating to some remarkablo colours produced by internal dispersion of light, which "must be reflected in cases of ordinary opalescence, from the faoos of minute pores in solids, or from particles of different densities, disseminated through solids, or suspended in fluids."—

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CHAPTER VIII.

Newton't Discoveries respecting the Inflexion or Diffraction oj LightPrevious discoveries of Grimaldi and Dr. HookeLabours of succeeding PhilosophersLaw of Interference of Dr. YoungFresneTs discoveriesThe Undulatory Theory generally.

Among the optical discoveries of Newton, those which he made on the Inflexion of Light hold a high place. They were first published in his Treatise on Optics in 1704, but we have not been able to ascertain at what period they were made. In the preface to that work, Sir Isaac informs us that the third book, which contains his experiments on inflexion, "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 we were then interrupted, and cannot now think of taking these things into consideration."

The phenomena of the inflexion 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.*

Having admitted a beam of the sun's light through a

* Phytico-MatlKsu de Limine, Coloribui, of hide, aliisque aniiexii. small pin-hole in a piece of lead or card into a dark chamber, he found that the light diverged from this aperture in the form of a cone, and that the shadows of all bodies placed in this light were not only larger than might have been expected, but were surrounded with three coloured fringes, the nearest being the widest, and the most remote the narrowest. In strong light he discovered analogous fringes within the shadows of bodies, which increased in number with the breadth of the body, and became more distinct when the shadow was received obliquely and at a greater distance. When two small apertures or pin-holes were placed so near each other that the cones of light formed by each of them intersected one another, Grimaldi observed, that a spot common to the circumference of each, or, which is the same thing, illuminated by rays from each cone, was darker than the same spot when illuminated by either of the cones separately; and he announces this remarkable fact in the following paradoxical proposition, " tJutt a body actually illuminated may become more dark by adding a light to that which it already receives." Grimaldi discovered also the beautiful phenomenon of the crested or curved fringes exhibited within the shadow of the rectangular termination of bodies.

"Without knowing what had been done by the Italian philosopher, our countryman, Dr. Robert Hooke, had been diligently occupied with the same subject. In 1672, he communicated his first observations to the Royal Society, and he then spoke of his paper as "containing the discovery of a new property of light not mentioned by any optical writers before him." In a valuable memoir read by him on the 10th of March, 1674, he not only described the leading phenomena of the inflexion, (or the deflexion of light, as he called it), but he distinctly announced the doctrine of interference,

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which has performed so great a part in the subsequent history of optics.*

Such was the state of the subject when Newton directed to it his powers of acute and accurate observation. His attention, however, was turnod only to the enlargement of the shadow of inflecting bodies, and to the three fringes adjacent to it. He was therefore led to take exact measures of the diameter of the shadow of a human hair, and of the breadth of the fringes at different distances behind it, and repeating these observations with light of different colours, he obtained two new and remarkable results:—(1.) That these breadths were not proportional to the distances from the hair at which they were measured; and (2.) That the fringes made in homogeneous red light were red, and the largest; that those made in violet light were violet, and the smallest; and that those made in green light were green, and of an intermediate size; the rays which formed the red fringe passing by the hair at a greater distance than those which formed the violet.

As already mentioned, when Newton made the preceding observations, he intended to repeat most of them with more care, and to make "some new ones to determine the manner how the rays of light are bent in their passage by bodies;" but having been then interrupted, he could not think of resuming the inquiry.

It is very difficult to ascertain his real views on the subject of inflexion. In his Discourse road in 1675, he ascribes it to the variable density of the ether within and without the inflecting body, thus regarding it as a new

* This doctrino is thus announcod. 1. That the same rays of light falliiig upon tho samo point of an object will turn into all sorts of colours by the various inclination of the object. 2. That colours begin to appear when two pulses of light are blended so well and so near together that the sense takes them for one.

species of refraction; and in his letter to Robert Boyle in 1679, he takes the same view of the subject, and considers the several colours of the fringes as produced " by that refraction." Pursuing the same idea, he asserts in the Scholium to the 96th Prop. of the first book of the Principia, that the rays of light, in passing near bodies, are bent round them as if by attraction; that the rays which pass nearest them are most bent, as if they were most attracted; that those which pass at a greater distance are less bent; and that those which pass at still greater distances are bent in an opposite direction.

In this remarkable passage Newton introduces, for the first time, the idea of a force bending the rays outwards; or of an iujlecting force bending the rays inwards, accompanied with a deflecting force bending them outwards. This opinion, however, he subsequently abandoned; for, in the third book of his Optics, he refers all the phenomena to a force which "bends the rays not towards, but from the shadow;" and he distinctly asserts, "that light is never known to follow crooked passages, nor to bend into the sltadow."

These erroneous opinions, now wholly exploded, arose from Newton's having never observed the internal fringes, or those seen within the shadow. Grimaldi had described them minutely in his work, and as they have been seen by almost every philosopher, it is not easy to explain how they should have escaped the notice of two such careful observers as Hooke and Newton. Without this cardinal fact, our author stumbled in his path, and was misled into the erroneous propositions that bodies act upon light at a distance; that this action bends its rays with a force diminishing with the distance; and that rays which differ in refrangibility differ also in flexibility. Nor was he nearer the truth when he conjectured in his third query that the rays of light, iu passing by the edges of bodies,

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