them, taking care to shut out all extraneous light from the room. If the light be yellow, as that emitted from a spiritlamp with a salted wick, a series of yellow and black bands will be seen on the glasses, the bright ones arising from the undulations propagated from one mirror to the other in such a manner that they are in the same phase (422), and the dark one when in opposite phases, and consequent production of interference. This experiment was first proposed by M. Fresnel. To 489. An interesting set of illustrations of the doctrine of luminous interference, is met with in the phenomenon of diffraction discovered by Grimaldi, a Jesuit of Bologna. observe these properly, a beam of diverging light is necessary; this may be obtained by making a small hole in a window shutter, and receiving the light on a screen at the distance of some feet. If a convex lens, of small focal length, be fitted in the hole in the shutter, the light is refracted almost to a point, from whence it diverges in a manner extremely fitted for experiments on diffraction. For small experiments, a pyramidal box, ABCD, about two feet long, and blackened inside, may be advantageously employed; at E, a convex lens, of an inch focus, is fixed, on which, by means of the plane mirror & (432), a sunbeam can be readily thrown. The light is refracted by the lens to a point, and then diverging, is received on a sheet of white paper placed at the bottom of the box; by means of a door shown at F in the section, the bottom becomes easily visible, without admitting any quantity of extraneous light. C G B F 490. If any small opaque bodies, as hairs, pins, &c., be held in the beam of diverging light, ELL, their shadows will be thrown on the bottom of the box, surrounded by coloured EXPERIMENTS ON DIFFRACTION OR INFLECTION. 337 fringes. If H be a section of a pin thus exposed, the fringes are seen surrounding its shadow, as though they were produced by coloured rays passing by its margin, not in straight lines, but in hyperbolic curves, as shown by intercepting them at different distances by a piece of card; when their decrease in extent will be found to be much more gradual than if the light passed by H in right lines. Besides their external fringes, there are internal ones within the shadow, which, if the body be narrow, as a pin, becomes completely filled with them. These colours are, as Lord Brougham has long ago shown, in harmonic proportion, like those of the solar spectrum (468). The tints of the coloured fringes, reckoning from the shadow, succeed each other in the following manner: 1st fringe-violet, indigo, blue, green, yellow, red. 3d fringe-pale blue, pale yellow, red. If homogeneous light (467) be employed, the fringes will be of the same colour as this light, and their intervals will appear black. The fringes are broadest in red, narrowest in violet, and of intermediate breadth in the other colours of the spectrum. 491. These phenomena admit of ready explanation on Dr. Young's theory of interference, for the diverging light which passes by one side of the pin (490), meeting with that passing on the opposite side, coincide, and produce a line of white light, which ought to occupy the middle of the shadow. Whilst those rays which differ in their paths, as those produced by undulations, which pass obliquely past the pin into its shadow meeting with those which pass more directly on the opposite side, being of course unequal in their paths, encounter under different phases (422), and interfere (486), either checking the undulation entirely, and producing Phil. Transactions, 1795. darkness, as when homogeneous light is used, or so partially checking it as to allow such a number of movements to be executed in a given time as shall be sufficient to produce a coloured fringe. 492. In shadows of this kind, formed by narrow bodies, the middle is always occupied by a luminous line, as though the light had passed directly through the centre of the diffractory body. This very curious fact is best observed by holding a small disc of metal on a slip of glass, in the diverging light (489); the rays passing by its circumference are inflected, and meet, after traversing equal paths, in similar phases in the centre of the shadow, producing a brilliant spot of light. The shadow thus precisely resembles that of a circular disc perforated in the centre. This beautiful experiment is best performed by means of a drop of thick black ink, or a mixture of lampblack and size, placed on a plate of glass, so as to form a circular spot about the tenth of an inch in diameter. For this modification of the original experiment of Fresnel, we are indebted to Prof. Powell. 493. If a disc, perforated with a very small hole in the centre, be held in the beam of diverging light (489), the converse of the last experiment will be observed; for those undulations which pass directly through the aperture, interfere with those passing more obliquely, and produce a dark spot on that part of the shadow corresponding to the hole in the disc. Thus, by the mutual interference of luminous undulations, we find light virtually changed to darkness, and darkness to light, by the discord or concord of the luminous waves. 494. If two knife edges be held very near each other in the beam of light (489), beautiful coloured fringes will be observed to border their shadow, and a dark line will, if they be sufficiently near, be seen to occupy the middle of the space, at which they are really separate. This result of luminous interference may be readily shown by fixing a slip of tin-foil on a plate of glass, and dividing it longitudinally; EXPERIMENTS ON DIFFRACTION OR INFLECTION. 339 very slightly separate the divided portions at one end, so that they may form a very acute angle with each other, as at ABCD. Hold this in the diverging light of the apparatus before described (489), about six inches from the bottom, so that it may form a well-defined shadow. The centre of the shadow, corresponding to the slit in the tin-foil, will be marked by an obscure line, and the shadow from this line will be covered with a beautiful set of fringes diverging from each other as they approach the apex of the acute angle F, formed by the foil, bounded on each side by hyperbolic curves, with their convex surfaces towards each other, as if diverging from vertices situated at EE. So that the widest parts of the curved fringes correspond to the apex of the angle formed by the slips of tin-foil. In the figure, FGG, represent the projection of the slit in the foil on the paper on which the shadow falls. This experiment is an easy and rough mode of repeating Newton's observations with the knife-blades.* 495. The phenomena of diffraction may be observed on a small scale in the manner recommended by Prof. Powell, by stretching a fine wire across, and in contact with a small lens; holding the outer surface next the eye, look through it at the light of a candle, admitted through a narrow slit. The dark image of the wire will be seen edged by the external, and the shadow marked by the internal fringes (490), in a very beautiful manner. Optice. Lib. iii., pars. i., obs. 10. 496. The explanation of the cause of colours by diffraction (491) is finely illustrated by placing a card on one side, and on a plane above or below the body н (489), so as to intercept some of the incident or diffracted light; the fringes then disappear, because one set of the undulations producing interference have been cut off. If a transparent body be substituted for the card, the fringes undergo a remarkable change, from the retardation of those undulations which are propagated through the transparent screen. 497. The brilliant tints of soap-bubbles, and thin plates of different transparent bodies, afford other examples of interference of light; for the undulations reflected from their first surfaces interfere with those reflected from the second (431*); and upon the amount of retardation thus experienced by the luminous waves, the varieties of colours observed in these thin plates depend. The colours of soap-bubbles are best seen by boiling a small quantity of soap with distilled water in a bottle, and corking it whilst boiling hot. The whole being secured from air, is allowed to cool, and on adroitly shaking the bottle, a large bubble, presenting the coloured bands with great beauty, may be readily formed; this bubble is permanent for several hours, and affords every facility for examining its tints. 498. The colours of thin plates of air may be observed by pressing a convex lens on a plate of glass, and holding it in the light, so that rays reflected from it will pass to the eye. At the point of apparent contact of the lens and glass, a black spot will, under these circumstances, be visible; this is surrounded by a great number of rings of different colours, each series of tints consisting of fewer colours as they recede from the centre. On holding the glasses between the eye and the light, a set of rings will be observed, differing in colour from those seen by reflection; and, complementary to them, each ring possessing that colour, which, by mixing with the tint of the corresponding reflected ring, would produce white light. |