CHAPTER VII.

NEWTON'S HYPOTHESIS OF REFRACTION AND REFLEXION-OF TRANSPARENCY AND OPACITY-HYPOTHESIS OF COLOURS-THE SPECTRUM SUPPOSED TO BE DIVIDED LIKE A MUSICAL STRING-INCORRECTNESS OF THIS SPECULATION-HOOKE'S OBSERVATIONS ON THE COLOURS OF THIN PLATES EXPLAINED BY THE VIBRATIONS PRODUCED IN THE ETHER BY THE LUMINOUS CORPUSCLES-HOOKE CLAIMS THIS THEORY AS CONTAINED IN HIS

MICROGRAPHIA-NEWTON'S RESEARCHES ON THE

COLOURS OF THIN PLATES-PREVIOUS OBSERVATIONS OF BOYLEHOOKE'S ELABORATE EXPERIMENTS ON THESE COLOURS-HIS EXPLANATION OF THEM-DR. YOUNG'S OBSERVATIONS UPON IT-NEWTON ACKNOWLEDGES HIS OBLIGATIONS TO HOOKE-NEWTON'S ANALYSIS OF THE COLOURS SEEN BETWEEN TWO OBJECT-GLASSES-CORRECTIONS OF THEM BY MM. PROVOSTAYES AND DESAINS-NEWTON'S THEORY OF FITS OF EASY REFLEXION AND TRANSMISSION-SINGULAR PHENOMENON

IN THE FRACTURE OF A QUARTZ CRYSTAL-NEWTON'S OBSERVATIONS

ON THE COLOURS OF THICK PLATES-RECENT EXPERIMENTS ON THE SAME SUBJECT.

IN the preceding chapter we have given an account of the first part of Newton's discourse on light and colours, read on the 9th December 1675, and explaining his hypothesis concerning ether and ethereal substances, and their effects and uses." In the second part of the portion read at the same meeting he proceeds to " the consideration of light" as connected with the supposed ether, that is to the cause of refraction, reflexion, transparency, and opacity.

Regarding the ether as more dense in free space than in solid bodies, and as diminishing in density towards their surface both from without and from within, Newton sup

poses the incurvation or bending of a ray of light, incident on such a surface, in one direction to produce refraction, and in another to produce reflexion, to be effected within "the space of ether's graduated rarity," or "physical superficies." In the case of refraction, from air to glass, the ray passes from denser into rarer ether, and is incurvated from the perpendicular in its passage through the physical superficies; whereas in reflexion from a dense medium, such as glass into air, it is incurvated upwards or towards the glass, and the incurvation may be such that the ray does not emerge but suffer total reflexion.

In order to account by the agency of ether for the simultaneous refraction and reflexion of light incident upon the same surface of glass or water, Newton supposes "that ether in the confine of two mediums is less pliant and yielding than in other places, and so much the less pliant (or more rigidly tenacious') by how much the mediums differ in density." When light therefore, that is small corpuscles, falls upon " this rigid resisting ethereal superficies, it puts it into a vibrating motion, so that the ether therein is continually expanded and compressed by turns." When a ray of light is incident upon it "while it is much compressed, it is too dense and stiff to let the ray pass through, and so reflects it; but the rays that are incident upon it at other times, when it is either expanded by the interval of two vibrations, or not too much compressed or condensed, go through and are refracted.”

When the ether is of the same rarity in every pore, or when the ether is evenly spread by its continual vibrations into all the pores when they do not exceed a certain size, the light will pass freely through the body, or the body will be transparent. But when the pores exceed a certain size, the density of the ether will be greater than that

which surrounds it, and the light being refracted or reflected at its superficies, the body will be opaque.

On the 16th December the second portion of Newton's discourse was read, in which he applies his hypothesis to the explanation of colours. For this purpose he supposes the particles of light to have different degrees of "bigness, strength, or power," red having the largest, and violet the least degree of any of these qualities. When light, therefore, is incident on the "refracting superficies," the smallest particles, namely, the violet, will be most incurvated or refracted, and the red the least; and when these fall upon the refracting superficies of the retina, they will there excite" the sensation of various colours according to their bigness and mixture, the biggest with the strongest colours reds and yellows, the least with the weakest blues and violets, the middle with green, and a confusion of all with white; much after the manner that in the sense of hearing, nature makes use of aerial vibrations of several bignesses to generate sounds of divers tones." Pursuing this idea, "the analogy of nature," he conjectures, "that colour may possibly be distinguished into its principal degrees, red, orange, yellow, green, blue, indigo, and deep violet, on the same ground that sound within an eighth is graduated into tones. In order to test this speculation by experiment, he forms a distinct spectrum, and, " because his own eyes are not very critical in distinguishing colour," he employs a friend to whom he has not communicated his thoughts, to measure the lengths of the different coloured spaces. The differences between the measures thus obtained, he says, "were but little, especially towards the red end, and taking means between these differences, the length of the image (reckoned not by the distance of the verges of the semicircular ends, but by the distance of the

centres of those semicircles, or length of the strait sides as it ought to be) was divided in about the same proportion that a string is between the end and the middle to sound the tones in the eighth."

Ingenious as this speculation is, it is contradicted by all the recent discoveries respecting the prismatic spectrum, of which we have given an account in a preceding chapter. It is not even true in the spectrum which Newton himself observed. There are not seven colours in any spectrum, and even if we divide it into such a number of parts, the divisions have no resemblance to those of a musical string.

From the explanation of colours produced by refraction, Newton proceeds to explain those produced by reflexion, namely, the colours of thin plates described by Hooke in his Micrographia. In order to do this, he supposes that the ethereal vibrations excited by a ray move faster than the ray itself, and so "overtake and outrun it, one after another." When light, therefore, is incident upon a thin transparent plate, the waves, excited by its passage through the first surface, overtaking it one after another, till it arrive at the second surface, will cause it to be there reflected or refracted according as the condensed or the expanded part of the wave overtakes it there. If the plate be so thin that the condensed part of the first wave overtakes the ray at the second surface, it must be reflected there; if double that thickness, so that the following rarefied part of the wave, that is, the space between that and the next wave, overtake it, there it must be transmitted; if triple the thickness, so that the condensed part of the second wave overtake it, there it must be reflected, and so where the plate is five, seven, or nine times that thickness, it must be reflected by reason of the third, fourth, or fifth wave, overtaking it at the second surface; but when it is

four, six, or eight times that thickness, so that the ray may be overtaken there, by the dilated interval of those waves, it shall be transmitted, and so on; the second surface being made able or unable to reflect according as it is condensed or expanded by the waves.

In this way he explains the coloured rings produced by pressing a convex lens against a plain glass; and he concludes this portion of his discourse, namely, his "Hypothesis," by applying it to certain phenomena of Inflexion or Diffraction, as observed by Grimaldi.

It was after the reading of this portion of his discourse that Hooke said, "that the main of it was contained in his Micrographia, which Mr. Newton had only carried farther in some particulars,”—a remark which led to the correspondence with Oldenburg and Hooke, which we have given in the preceding chapter.

In the remainder of his discourse, Newton gives an account of his beautiful experiments on the colours of thin plates; but before we enter upon their consideration, we must notice the previous observations of Boyle and Hooke, in order that we may apportion to Hooke and to Newton the discoveries which they actually made. In the details into which this will lead us, we shall see two great minds striving for victory,-calling forth all their powers to surmount the difficulties which beset them in their path,—deviating from the rigorous process of research which both of them recognised, and perhaps forgetting, in the ardour of their pursuit, some of those courtesies which are now deemed essential in intellectual warfare.

In his book on Colours,1 Mr. Boyle informs us, that divers, if not all essential oils, as also spirit of wine, when

1 Experiments and Observations touching Colours. Exp. xix. p. 243. London, 1664.