EXPERIMENT XIV.

If the concave prismatic lens be substituted for the convex, the braid or beam of light passing through it will diverge immediately from the centre of the prismatic lens over a cone of shade forming the iris as before. These effects may be rendered visible in a very beautiful manner by the smoke of a sheet of brown paper burning without flame,or by the dust of a powder-puff shaken in the light near its egress from the prism.

the

REMARKS. These phenomena of transmitted light indicate the effects of other figures of the lenticular prism: they elucidate also the powers of optical glasses in general, and throw light upon phenomena of colored rings, observed by Sir Isaac Newton be tween two object-glasses laid upon one another; for the figure of Spheric Lenses may be considered as comprehending an infinity of lenticular-prismatic figures, in the same manner as the circle comprehends an infinity of triangles, &c. Hence there is a double circular refraction in the incumbent lens, and a like reflection in the recumbent lens concurring in the production of the colored rings; and hence the phenomena of spherical and prismatic lenses admit of a similar explanation.

EXPERIMENT XV.

Admit a beam of the sun into a darkened chamber through a scioptic ball, or a circular opening in the window-shutter of the diameter of the lenticular prism, and first removing the small tube of the chromascope, so adjust it in the line of the sun-beam, that the light may pass through it in the manner of Experiment XIII., when a magnificent circular iris of colored bow will be cast upon the opposite wall of the apartment, or a magnitude proportionate to the size of the room and distance from the chromascope, and of a brilliancy unexampled even in the solar rainbow itself."

A large white screen upon a horizontal axis, is of great advantage in these experiments to receive the solar spectrum at different angles and distances. Upon the reverse of the screen, large circles and different objects may be formed concentrically for Experiment VIII., &c. The present experiment affords a method by which a rainbow, of any arc or diameter, may be superinduced upon a picture, into which the artist may design to introduce the phenomenon, so as to try its effect and the best way of producing it.

REMARKS. Of that most beautiful phenomenon the rainbow,

'Optics, Book 11. Part 1.

;

none of the explanations hitherto offered can be pronounced universally satisfactory. The received hypothesis of the refraction of the solar rays in single spherical drops of rain, rapidly descending, dividing, subdividing and dispersing as they fall through the atmosphere, is, notwithstanding the rich mathematical dress that envelopes it, remote from the light of demonstration. Is it not more consonant to nature and experience that the bow should be produced, as in the lenticular prism, by the refraction of light in the entire mass of rain, than in its isolated particles? It is a law of optics that when light passes from a rarer to a denser medium it is thus refracted and it is well known that the rainbow is produced by a partial shower invariably opposed to the sun; and that it never exceeds a semi-circle. Partial rain, in its descent through the atmosphere, is progressively accelerated and resisted; hence we may infer that the shower in its fall takes a form nearly hemispherical, and that it is densest at the centre, the whole of which is favorable to the refraction and reflection of the sun's rays in the form of a rainbow from the entire mass of rain and sky to the eye of an observer placed between them and it accords herewith, that no sooner is the observer involved in the medium of refraction, by the rain becoming general, or passing between himself and the sun, than the bow disappears altogether; while it would be difficult, upon the theory of the drop, to determine why we have rain without a bow.1

However these things be, it remains an object of importance to the naturalist to demonstrate the true cause of a phenomenon so enchanting, that he who is unaffected by its beauty, appears to be worse than inanimate, since the poet, in language highly figurative, yet naturally just, has sung, that when

Iris her lucid various bow on high

Gaily displays, and soothes the weeping sky,
The boist'rous winds are hush'd in deep amaze,
And ocean stills his angry waves to gaze!

EXPERIMENT XVI.

If the last experiment be repeated with a beam from the moon shining through a clear atmosphere, in her second or third quarter,

1 In coincidence herewith, the following notice has lately appeared in the Journal of the Royal Institution, No. XIII. "A new theory of the common rainbow has been advanced by Dr. Watt, of Glasgow, by which he accounts for the effect without refraction in small drops of water. The rainbow, it is stated, is frequently seen toward the quarter where no rain is falling, and where of course it cannot be caused by drops; but it is observed, that the edge of a cloud is always at those times between the observer and the sun; and Dr. W. attributes the rainbow to the beams of the sun refracted by this edge and thrown on the dark sky opposite, which receives them as a screen.' 'There are, however, many phenomena v hich cannot be explained in this way, the bow formed in the mist of a cataract,' &c.-' and where the shower is seen falling,'-'must be accounted for by refraction in drops."

in place of the solar ray, a similar iris of peculiar faint colors is produced, affording a ready mode of exhibiting the rare and interesting phenomenon of a lunar rainbow.

EXPERIMENT XVII.

The magnitude of the spectra in these latter experiments renders them particularly advantageous for the performance of experiments in or upon transient or prismatic colors. Thus, if a person be so placed that the solar bow of Experiment XV. cross his eyes, so that the blue of the spectrum fall upon one eye and the yellow upon the other, if he then close the eye upon which the blue falls, and look with the other eye at the chromascope, he will perceive a yellow light; then opening the former eye and closing the latter, he will perceive a blue light; but, finally, if he keep his position, and open both eyes, he will perceive a green light only, demonstrating the concurrence of the two former colors and both organs in the joint sensation of green. This experiment extends to other cases of the composition of colors.

EXPERIMENT XVIII.

Let a person be placed, as in the last experiment, in the broad spec trum of the chromascope, so that the red fall distinctly in one eye and the green in the other; if then each eye be alternately shut while the other remains open, the color respectively shining upon each organ will be seen alternately; yet if both eyes be opened together no color will appear. By means of a small mirror held in the hand, the experimenter may perform these experiments upon himself, without an assistant, but, in either case, the spectator should be placed at a distance from the chromascope, to be governed by the breadth and coloring of the spectrum.

REMARKS. This experiment extends also to other cases, and demonstrates the neutralising, extinguishing, complementary, contrasting or compensating power of colors, which may justly be considered as the key to chromatic science. In whatever way this neutralisation of colors is produced, there is an union or coalescence of the primary triad, blue, red and yellow, in due subordination; and it is remarkable, that whenever it arises from the union of two colors, as in the present experiment, these colors bear the relation of that interval in music which is called the fourth, corresponding to the diatessaron of the ancients, which was held by them to be the concord upon which all others depend. The harmonising power

of colors renders it evident that it was not without some foundation. VOL. XVII. NO. XXXIII.

Pam.

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Father Castel attempted to construct an ocular harpsichord, for the purpose of exhibiting to the eye a pleasurable sensation from colors, analogous to that which the ear receives from musical sounds; and he did not succeed, because his scale was erroneous, and because the harmonious effects of colors are not merely successive or temporary, like those of sound, but they are also co-expansive, or in space.

EXPERIMENT XIX.

On a clear evening when the moon is at the full, remove the small tube, of the chromascope, and so adjust it, with the open end toward the moon, that it may be viewed through it by placing the eye close to the lenticular prism at K, Fig. 14, when the moon will appear refracted into a beautiful lucid bow, the colors of which will not be at all inferior in brilliancy to the solar spectrum, Experiment XV.

EXPERIMENT XX.

Any dark evening when the sky is clear, the planets and fixed stars may be viewed in the manner of the above experiment, when the light of either will be refracted into a colored bow, differing from that of the last experiment only in breadth and the faintuess of its colors.

REMARKS. The two latter experiments demonstrate, that the light of the fixed stars and of the primary and secondary planets differs not, by the analysis of refraction, from that of the sun: a question which has been considered of some importance. In the same manner may be examined the light of the glow-worm, lightning, the phosphori, &c.

EXPERIMENTS XXI., &c.

If day-light be admitted into a darkened chamber through a small round aperture in the window-shutter, and it be viewed, as in the last experiments, at any convenient distance, a brilliant circular iris will be produced, as in those experiments; for this purpose a circular piece of lead-paper or thin sheet-lead, perforated with the opening required, and fitted to the socket of a scioptic ball, is very well adapted; and the plano-convex or conical shape of the lenticular prism is very appropriate to these and other experiments, in which the object is viewed through the prism, because by placing the eye close to the plane side and the apex of the cone in a di

rection concentrical to the object, a wider field of vision is commanded.

REMARKS. In the manner of the present experiment, the first twelve of the preceding experiments, and many others, may be performed by transmitted light, by means of objects perforated in thin sheet-lead and adapted to the socket of a scioptic ball, or similar opening in a window-shutter; and these experiments differ from the former principally in the circumstances of superior brilliancy of the spectra and the inverted order of their colors.

To describe all the experiments of which this instrument is capable would be impossible; the foregoing are, however, sufficient to illustrate its powers, and to conduct to others. What is here performed by the refraction of lenticular prisms may be also accomplished by the reflection of similar mirrors; add to which the other figures of the lenticular prisms before indicated, and a similar variety of annular prisms, and the variation of which the chromascope is susceptible, from the simple hand-glass to its combination with other optical instruments, and a new and wide field is opened for dioptrical experiment, adapted equally to instructive amusement and the advancement of science.

Upon the whole it has appeared, that whether we examine colors experimentally by the composition of the inherent or reflected colors of pigments, or by the transient or refracted colors of the prism they still present the same uniform relations; such also is the result of a like investigation of the transmitted colors of transparent liquids, by an instrument constructed for the purpose, of hollow glass wedges, filled with colored liquids, and graduated so as to denote the quantity of color in the light transmitted at each division of the wedges. By means of this instrument we are enabled to compound braids of colored light in an achromatic or neutral state, and at the same time to determine numerically the quantity of each primary, &c. which enters into the composition of the colorless ray. We are enabled by the same instrument to ascertain the proportion of each of the primaries which constitute any given hue, shade, or tint of color, &c. and have therefore denominated it a chromameter or color-gauge, of which only by the way, having already protracted these remarks, &c. beyond the proper limits of an Appendix.