to the other side, or that next the eye, without changing its polarity. According to this view, the dark tints ought to be on the central aspects of fringes. Or, if we suppose that the incident pencil is negative, and that its own polarity is transmitted in a continuous stream to the surface of the crystalline lamina next the eye, where the two poles are contiguous, it will, in like manner, give a negative tint. In cases where chromatic axes are developed in nature, and seen in common light, one of the parts of the incident ray seems to illuminate or excite them, and the other to render them visible.

The colours of thin plates generally, then, are presented in two series. One is developed simply on account of the thinness of the plate, which becomes compatible with a single chromatic axis. This display is confined to the most attenuated laminæ, is visible in common light, and is analogous to the colour of most natural bodies. Supposing that the lamina is a fragment of a polarizing crystal, after it has attained too great a thickness for the institution of a single chromatic axis, it loses its opacity and colour, and simply transmits common light, not much affected by it. But when its thickness is too considerable for its becoming coloured in virtue of its thinness, it begins to repeat the same series of tints as it grows thicker, in consequence of its double refraction. Its thickness, which destroyed the first series, is the means of developing the second. The first series, then, may be represented by coloured laminæ, held at right angles to the optical axis, or with their disks fronting the eye; the second, by the same laminæ held in the plane of the optical axis, or with their edges fronting the eye. In the first series, we see only one pole at a time; in the second, by that apparatus which has been mentioned, we may see both as we should do were our eyes good enough, when the edge of a thin plate, coloured by its thinness, is turned towards the eye.

If rr (Fig 31.) be the thickness of a thin plate, which gives certain tints in natural light, the doubly refracting plate, A B, to give the same tints, must have a thickness equal to the product of the thin plate, and the cotangent of separation between the two rays, at a vertical incidence, possessed by the

crystalline lamina. Or, calling the angle of bifurcation proper to the lamina λ, the thickness of the idiochomophanous plate t, and that of the doubly refracting plate, which gives the same Thus it appears

tints in singled light, T: then T

t

tang. λ.

that the colours of crystalline plates do not depend absolutely on their thickness, as is the case with thin plates, but on this element, combined with their doubly refracting power; and hence a plate of a very weak doubly refractive intensity may become coloured, though its thickness be very great.

The weakest sort of polarizing action seems to be that of certain liquids, and even vapours, such as turpentine and syrup, inclosed in tubes; and, among mineral bodies, that which takes place along the axis of quartz. Not only is the incident ray not bifurcated, but it requires a considerable thickness to transverse it, which must, of course, be greater in proportion as the directive force of the incident tint is greater. The next degree appears to be that in which the plate simply transverses the incident pencil, as is the case when a rhomboid of calcareous spar is applied to a ray singled by reflection, with its principal section parallel to the incident edges; and the highest degree is that in which the incident pencil gives rise to two, which, when of equal intensity, are transverse, or complementary to each other.

Such are some of the most interesting phenomena presented by thin doubly refracting laminæ, in which the polarizing force is equal in intensity all over the plate, except in the neutral axes. But in plates possessing individuality of form, such as equatorial laminæ, taken from crystals which are not tessular, in planes at right angles to the axis, other phenomena become visible, which possess the most beautiful and interesting characters. In fact, in viewing such a lamina in singled light, we are viewing the base of the ray of light proper to the symmetrical medium examined; and a state of subtile matter, analogous to that which it has been presumed a ray of the radiant medium possesses, is exhibited before the eye in the most vivid tints, determined by the medium to which it is attached, but always retaining the same generic features. In

the hope that the molecular structure of many bodies will soon be determined, this subject is now omitted, with the confident anticipation of the pleasure which any one will derive, who chooses to apply the principles of the structure of matter contended for in this work to all the phenomena of physical optics. Analogous phenomena of still greater splendour may be developed by the partial distribution of heat in transparent structures, not naturally possessing free polarization or individuality of form. Thus, let a rectangular plate of annealed glass be examined in singled light, and it will exhibit no traces of polarization; but while it remains illuminated, and viewed with singled light, let one of its edges be applied to a hot body; "nearly at the same instant," the light of the surface contiguous to the hot body is thrown into a state of polarity; a perfect image of that state is speedily developed at the other edge or pole, and the middle, or equatorial region, contains the consecutive polarity. If a diamond cut be drawn through the equator, so as to divide the plate into two, though they be in their former position, there are now two polarized forms which are images of each other; and, by using pieces of glass of different forms, and crossing plates upon each other, the most strikingly beautiful figures are displayed, all of them perfectly illustrative of the habitudes of subtile matter, whose polarity is excited. If a model of a human brain were made of crystal, with ramifying tubes distributed internally for applying heat to it irregularly, should we not observe most interesting phenomena ?

But, not only may glass, by an unequal distribution of heat in it, acquire an individual polarity or circulation, according to the form which it happens to possess; the radiant medium also, like any other transparent tessular medium, may be brought into the same condition, and the forms and tints. which are developed are still more beautiful. Most admirable attempts have been made to explain them, solely on the principles of a mechanical interference of waves in an undulatory medium. What could baffle a genius like that of Fresnel? He always, when he errs, goes beyond rather than falls

short of the truth. But, to dispense with half an interval at pleasure, is to dispense with all that ever can be required to make any phenomenon conform to the hypothesis of recurrent colours; and when, in addition to this, any one of many regions may be fixed upon as fit places for interference, it is not to be wondered if this doctrine has been extended to phenomena where it does not apply. The principle of the chromatic axis seems to me to act to a great extent in producing the brilliant spectra of a diffracted sunbeam. The phenomena will be alluded to at the end of the chapter on Radiant Heat.

OF VISION.

THE apparatus by which the condition of external objects, as to illumination, is announced to the mind, is the Eye, an optical instrument of the most perfect kind, the aplanatic structure of the lens of which it has been the ambition of opticians to imitate, though hitherto with only partial success. Like tessular bodies, and such as want individuality of form, the eye acts upon light very passively, at least during life; merely conducting without change, like the radiant, which is contiguous to it, the lumeniferous excitement emanating from the objects before it. In consequence of its lenticular structure, however, the direction of the incident rays is changed, and, within the range of distinct vision, they are made to converge to a focus on the interior wall of the eye. There an image of external objects is formed; and, in an eye examined by cutting in upon the back of it, this image may be seen painted in its natural colours, as in a camera obscura. Whether the region where the image is formed possesses opacity enough, during the fluidity of life, to intercept and form a base to the rays meeting there, cannot be discovered. The wall, or cup, which receives the incident rays, is covered by a stratum of cerebral matter, named the Retina, which, in man, and most animals, reposes on a very opaque pigment. This pigment seems to perform a most important function in vi

sion; and its colour is the index of that state of external lumeniferous excitement, which cannot produce vision. Being itself composed of radiant matter in a condensed state, this pigment must possess the light proper to the atoms of which it consists, in a certain state of excitement; and, when the state of the excitement of the external radiant matter is the same, no re-action can take place between them, it must be the same as to vision as if the eye were closed. The pigment of the human eye, when looked upon (other objects being excluded), excites the same sensation as the closing of the eye or the radiant medium at night. The optic nerve remains unchanged, and in the natural state of its illumination. Both night and the pigment are black. The state of their illu mination is the same. If, instead of being black, it were white, that is, in an opposite state of illumination to what it is now, and to that of the radiant medium at night, the degree of radiant excitement fit for vision would undergo a corre sponding change. In these circumstances, dark tinted, and obscure bodies, would be most distinctly visible; and the night would be the season best fitted for distinct vision. Now, it is remarked of diurnal animals generally, that they agree with man in having dark or nocturnal pigments; of nocturnal animals, it is remarked that the pigments of their eyes are lightcoloured or diurnal. Each has a state of vital lumeniferous excitement, consecutive to that of the radiant medium, during the hours when its vision is most distinct. The lumeniferous excitement of the radiant medium during night, however, being much less considerable than during day, nocturnal animals have the whiteness of their pigments supplemented by large organs of vision, and often a lucid stratum on which the retina reposes. This lucid stratum, in that region in which it remains uncovered by the pigment, and is more particularly called the tapetum, displays a metallic lustre, which always Indicates abundance of superficial light; and that its light is differently excited from that of the radiant medium by night, and that of our eyes, is shewn by this-that, in the darkest. place, where every thing else is invisible, by the light of its lucid tapetum, a nocturnal eye, such as that of a cat, may be