Newton found, on making these calculations, that the distance between the glass surfaces where the second red circle was formed was double the distance corresponding to the first; that at the third red circle the distance was triple that of the first, and so on. Of course it followed that wherever the dark rings were formed, the distances between the glass surfaces were not an exact number of times the space corresponding to the first red circle.

Newton perceived that these phenomena were the direct manifestation of those effects which corresponded to the breadth or amplitude of the waves of light in the undulatory theory, although he used the corpuscular nomenclature. The space between the surfaces of glass at the first red ring was the breadth of a single wave, the space at the second red circle the breadth of two waves, and so on. Within the first red circle the space between the glasses being less than the breadth of a wave, the propagation of the undulation was stopped, and darkness ensued; in like manner, in the space corresponding to the second dark ring, the distance between the glasses being greater than the breadth of one wave, but less than the breadth of two, the propagation was again stopped, and darkness produced. But at the second red circle, the space being equal to the breadth of two waves, the undulations were reflected, and the red ring produced, and so on.

It then became evident that, to measure the breadth of the red waves, it was only necessary to calculate the distance between the glasses at the first red ring.

Number of Waves or Undulations in an Inch.—When light of other colours was let fall upon the glass, a similar system of luminous rings was produced, but it was found in each case that the first ring varied in its

diameter according to the colour of the light, and therefore that the breadth of the waves of lights of different colours is different. It appeared that the waves of red light were the largest; orange came next to them; then yellow, green, blue, indigo, and violet succeeded each other, the waves of each being less than those of the preceding. But the most astonishing part of this investigation was the minuteness of these waves. It appeared that the waves of red light were so minute, that 40,000 of them would be comprised within an inch, while the waves of violet light were so small that 60,000 would be contained within an inch; the waves of light of other colours were of intermediate magnitudes.

Table of Undulations.-In the annexed table are given the length of the waves of each prismatic colour, the number of them which measure an inch, and the number of waves, pulsations, or undulations per second which strike the eye.

In this table, which was calculated by the eminent Dr. Young, the numbers of waves or undulations per second are given in round numbers, so as to render the principles of the investigation as intelligible as possible.

The results contained in the table can scarcely fail to excite in us sentiments of the greatest wonder and astonishment. It is well known that solar light moves at the rate of about 200,000 miles per second; it necessarily follows that a ray of light 200,000 miles in length must enter the pupil of the eye each second, and as the perception of light and colour is produced by pulsations of the membrane of the eye vibrating in accordance with each ethereal undulation or wave propagated from a visible object, whenever we behold a red object, the retina, or membrane, of the eye trembles or pulsates upwards of 477,000,000,000,000 times every second. For each of the other colours of the spectrum the number of vibrations the eye makes in a second is still greater; when violet light is perceived it trembles at the rate of about 720,000,000,000 times in a second.

That man should be able to measure with certainty such almost infinitely small portions of time and space is most wonderful; for it may be observed that whether we adopt the corpuscular theory of light, according to which the molecules of light are supposed to be endowed with attractive and repulsive forces, to have poles to balance themselves about their centres of gravity, and to possess other physical properties, or adopt the undulatory theory, the periods and spaces just given have a real existence.

It is not unreasonable to suppose that the heat rays, and chemical, or actinic, rays, which accompany the luminous rays in the solar beam, are endowed with properties analogous to those of the luminous rays, and possess qualities no less wonderful.

Inflexion or Diffraction of Light.-The property of light called inflexion, or diffraction, was first discovered

by Grimaldi in 1665. Since that time the subject has received a good deal of attention from many eminent philosophers, but it is to M. Fresnel that we are indebted for the most successful investigation of the phenomena. If the rays of light diverging from a luminous point F (Fig 42) fall upon an opaque object A B, all those rays included within the angle AF B

Fig. 42.

will be intersected, so that a screen held at C D will receive none of these rays. If we produce the lines F A and FB to A and B', they will include upon the screen those spaces which would have been illuminated by the rays proceeding from F, which are stopped by the opaque body A B. All the rays included in the angles A F C and B FD will proceed uninterruptedly, and will fall upon the screen. If these rays suffered no change of direction, they would illuminate those portions of the screen included between c and A', and D and B'. There would by this means be a well-defined shadow of the object, a B, formed upon the screen at A'B', and the rest of the screen would be illuminated in the same manner as it would have been if the opaque body, A B, had not been present.

It is found by experiment that no such exact and well-defined shadow of the opaque object would be formed upon the screen. The outline of the space which would limit an exact and geometrical shadow of A B being determined, it is found that within this space light will enter, and that outside this space the illumination is not the same as it would have been if the object, a â, had not been interposed.

Hence it is inferred that the rays of light which pass

H

the edges of the opaque object do not proceed in the same straight lines, A A and B B', in which they would have proceeded if the opaque object was not present. The edge of the shadow is not a well-defined line, separating the illuminated from the dark part of the screen, but a line of gradually-decreasing brilliancy from the illuminated part of the screen to that in which the shadow becomes decided.

The effect produced by the edges of an opaque body upon the light passing in contact with them, by which the rays are bent out of their course, either inwards or outwards, is called inflexion or diffraction.

This phenomenon is considered as a consequence of the general property of undulation. When the system of waves propagated round r as a centre encounters the obstacles A B, subsidiary systems of undulation will be formed round A and B respectively as centres, and will be propagated from those points independently of, and simultaneously with, the original system of waves whose centre is F, and which will also proceed towards ca and DB'. In a certain space round the lines A a' and B B', along which the rays, grazing the edge of the opaque body, would have proceeded, the two systems of undulation will intersect each other and produce the phenomena of interference.

The Law of Interference.-If two pencils of light, radiating from two points close to one another, fall upon the same spot of a piece of paper, in which case they may be said to interfere with one another, for if the paper were removed they would cross one another at that point, then if the lengths of their paths, or the distances between the paper and the two radiant points are the same, they will form a bright spot or fringe of light, having an intensity greater than that which