continues to cd, all the light which falls upon the portion nm will be reflected, and none transmitted; and so on, the light being transmitted at mE and pn, and reflected at nm and qp. Hence to an eye above E the wedge-shaped film of which AEC is a section will be covered with parallel bands or fringes of light separated by dark fringes of the same breadth, and they will be all parallel to the thin edge of the plate, a dark fringe corresponding to the thinnest edge. To an eye placed below CE similar fringes will be seen, but the one corresponding to the thinnest edge mE will be luminous.

If the thickness of the plate does not vary according to a regular law as in fig. 9, but if, like a film of blown glass, it has numerous inequalities, then the alternate fringes of light and darkness will vary with the thickness of the film, and throughout the whole length of each fringe the thickness of the film will be the same.

We have supposed in the preceding illustration that the light employed is homogeneous. If it is white, then the differently coloured fringes will form by their superposition a system of fringes analogous to those seen between two object-glasses, as already explained.

The same periodical colours which we have now described as exhibited by thin plates were discov ered by Newton in thick plates, and he has explained them by means of the thecry of fits; but it would lead us beyond the limits of a popular work like this to enter into any details of his observations, or to give an account of the numerous and important additions which this branch of optics has received from the discoveries of ceding authors.

CHAPTER VII.

Newton's Theory of the Colours of Natural Bodies explained-Objec tions to it stated-New Classification of Colours-Outline of a New Theory proposed.

IF the objects of the material world had been illuminated with white light, all the particles of which possessed the same degree of refrangibility, and were equally acted upon by the bodies on which they fall, all nature would have shone with a leaden hue, and all the combinations of external objects, and all the features of the human countenance, would have exhibited no other variety but that which they possess in a pencil sketch or a China-ink drawing. The rainbow itself would have dwindled into a narrow arch of white light,-the stars would have shone through a gray sky, and the mantle of a wintry twilight would have replaced the golden vesture of the rising and the setting sun. But He who has exhibited such matchless skill in the organization of material bodies, and such exquisite taste in the forms upon which they are modelled, has superadded that ethereal beauty which enhances their more permanent qualities, and presents them to us in the ever-varying colours of the spectrum. Without this the foliage of vegetable life might have filled the eye and fostered the fruit which it veils,-but the youthful green of its spring would have been blended with the dying yellow of its autumn. Without this the diamond might have displayed to science the beauty of its forms, and yielded to the arts its adamantine virtues ;-but it would have ceased to shine in the chaplet of beauty, and to sparkle in the diadem of princes. Without this the human countenance migăt

have expressed all the sympathies of the heart, but the "purple light of love" would not have risen on the cheek, nor the hectic flush been the herald of its decay.

1

The gay colouring with which the Almighty has decked the pale marble of nature is not the result of any quality inherent in the coloured body, or in the particles by which it may be tinged, but is merely a property of the light in which they happen to be placed. Newton was the first person who placed this great truth in the clearest evidence. He found that all bodies, whatever were their peculiar colours, exhibited these colours only in white light. When they were illuminated by homogeneous red light they appeared red, by homogeneous yellow light, yellow, and so on, "their colours being most brisk and vivid under the influence of their own daylight colours." The leaf of a plant, for example, appeared green in the white light of day, because it had the property of reflecting that light in greater abundance than any other. When it was placed in homogeneous red light, it could no longer appear green, because there was no green light to reflect; but it reflected a portion of red light, because there was some red in the compound green which it had the property of reflecting. Had the leaf originally reflected a pure homogeneous green, unmixed with red, and reflected no white light from its outer sur face, it would have appeared quite black in pure homogeneous red light, as this light does not contain a single ray which the leaf was capable of reflecting. Hence the colours of material bodies are owing to the property which they possess of stopping certain rays of white light, while they reflect or transmit to the eye the rest of the rays of which white light is composed.

So far the Newtonian doctrine of colours is capable of rigid demonstration; but its author was not content with carrying it thus far: he sought to

determine the manner in which particular rays are stopped, while others are reflected or transmitted; and the result of this profound inquiry was his theory of the colours of natural bodies, which was communicated to the Royal Society on the 10th February, 1675. This theory is perhaps the loftiest of all his speculations; and though, as a physical generaliza. tion, it stands on a perishable basis, and must soon be swept away in the progress of science, it yet bears the deepest impress of the grasp of his powerful intellect.

The principles upon which this theory is founded are the following:

1. Bodies that have the greatest refractive powers reflect the greatest quantity of light; and at the confines of equally refracting media there is no reflection.

2. The least particles of almost all natural bodies are in some measure transparent.

3. Between the particles of bodies are many pores or spaces, either empty or filled with media of less density than the particles.

4. The particles of bodies and their pores, or the spaces between the particles, have some definite

size.

Upon these principles Newton explains the origin of transparency, opacity, and colour.

Transparency he considers as arising from the particles and their intervals or pores being too small to cause reflection at their common surfaces,* so that all the light which enters transparent bodies passes through them without any portion of it being turned from its path by reflection. If we could obtain, for example, a film of mica whose thickness does not exceed two-thirds of the millionth part of an inch, all the light which fell upon it would pass through it, and none would be reflected. If this film was then

* Optics, Book ii. Prop. iv,

cut into fragments, a number of such fragments would constitute a bundle, which would also transmit all the light which fell upon it, and be perfectly transparent.

Opacity in bodies arises, he thinks, from an opposite cause, viz. when the parts of bodies are of such a size as to be capable of reflecting the light which falls upon them, in which case the light is "stopped. or stifled" by the multitude of reflections.

The colours of natural bodies have, in the Newtonian hypothesis, the same origin as the colours of thin plates, their transparent particles, according to their several sizes, reflecting rays of one colour, and transmitting those of another. "For if a thinned or plated body which, being of an uneven thickness, appears all over of one uniform colour, should be slit into threads, or broken into fragments of the same thickness with the plate or film, every thread or fragment should keep its colour, and consequently, a heap of such threads or fragments should constitute a mass or powder of the same colour which the plate exhibited before it was broken: and the parts of all natural bodies being like so many fragments of a plate, must, on the same grounds, exhibit the same colour."

Such is the theory of the colours of natural bodies, stated as clearly and briefly as we can. It has been very generally admitted by philosophers, both of our own and of other countries, and has been recently illustrated and defended by a French philosopher of distinguished eminence. That this theory affords the true explanation of certain colours, or, to speak more correctly, that certain coloms in natural bodies are the colours of thin plates, cannot be doubted; but it will not be difficult to show that it is quite inapplicable to that great class of phenomena which may be considered as representing the colours of natural bodies.

The first objection to the Newtonian theory is the

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