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hot glass of uniform temperature be unequally cooled, as by placing one of its edges on a cold mass of iron, it acquires doubly refracting properties, and when examined by polarized light, presents fringes, &c. similar to those observed in glass unequally heated. It is obvious, however, that as the physical condition produced by cold is diametrically opposite to that caused by heat, so the structure of the corresponding parts of the two glasses (the one which has been cooled, and the other which has been heated) must be opposite.

4. Unannealed glass is a double refractor.—If glass be suddenly cooled after having been melted, it possesses certain remarkable properties, which unfit it for ordinary use. Sometimes it splits or flies to pieces in the act of cooling; or if it fail to do this, a very moderate change of temperature, a slight external force, a scratch, or a slight fracture, is sufficient to cause it to crack and fly to pieces. The glass tears called Rupert's drops, or hand-crackers, and the proofs, or philosophical phials, are familiar illustrations of this kind of glass. In order to prevent it acquiring this remarkable condition, glass, after being fashioned, is submitted to the process called annealing, that is to very slow cooling in the annealing oven or Her. Glass which has undergone this process is said to be annealed, while that which has not is termed unannealed. But the so-called unannealed glasses sold by the opticians are in fact annealed glasses, which have been reheated until they begin to soften, then cooled by placing them on the ashes beneath the furnace, and afterwards ground and polished.

The optical properties of unannealed glass are very remarkable. To comprehend them let us consider the mechanical condition of the glass. When a mass of red-hot and soft glass is exposed to a cool air, its external portion becomes cold and rigid, while the inner parts are still hot and soft. After a short time, however, the latter solidify and cool, but are prevented from contracting themselves into the smaller bulk which is natural to them in their cooled state, by the rigid crust, which acts like an arch or vault, and keeps them distended, but which is to a ceitain extent strained and drawn somewhat inwards by the tension exercised on it by the internal parts. It is obvious then that the different parts of a mass of unannealed glass are unequally and differently strained; the internal being in a state of distention or dilatation, the external in that of compression. So that the state of the different parts, and the distribution of the forces, will be almost exactly similar to those already described, in the case of annealed glass which has been unequally heated. "The analogy between the cases," says Sir John Herschel, "would be complete, if, instead of supposing the annealed plate heated at one edge only, the heat were applied to all the four simultaneously, by surrounding it with a frame of hot iron."

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There is one very important point in reference to these unannealed glasses, to which I must beg your attention; I refer now to the circumstance that in them, the polarizing (doubly refracting) structure depends entirely on the external form of the glass plate, and on the mode of aggregation of its particles. This will be very obvious by observing the different shapes of the fringes respectively presented by square, circular,oval, rectangular, and other shaped plates. The circular and square plates have only one axis of [no] double refraction; whereas the oval and rectangular plates have two axes. By dividing and subdividing these plates, the doubly refractive property is not only greatly diminished, but sometimes even destroyed, if the portion be very small. Moreover, it is distributed in a new manner, according to the shape of the fragment. The dissected unannealed glasses, sold in the opticians' shops, beautifully illustrate the dependence of the form of the coloured fringes on the external shape of the glass. Thus the pattern produced by one circular piece of unannealed glass, is very different to that of a circle formed by joining four segments.

In these particulars, the unannealed plates of glass differ very widely from doubly refracting crystals. The fringes and colours, presented by the latter, are unaltered by the changes we may effect in the external form of the crystal—the smallest fragment producing the same system of fringes as the largest; and, provided the thickness remains the same, the polarizing force suffers no diminution by the reduction in size.

We are then constrained to infer that the optical properties of crystals are those of their integrant molecules; while those of the unannealed glasses depend on the mode of arrangement of the molecules, and on the external form of the mass.

The effects produced by superposing similarly shaped pieces of unannealed glass are striking, and, at first, surprising; but, on consideration, may be easily understood. If they be symmetrically superposed, similar points being laid together, the tints will be equal to the sum of the separate tints:—but, if superposed crosswise, the resulting tints will be the difference of the separate tints. This may be conveniently shown by causing an unannealed glass bar to rotate in front of another unannealed bar.

Applications.—These facts respecting the properties of unannealed or imperfectly annealed glasses, admit of some valuable practical applications. To the optician it is of the highest importance that the glass, of which lenses and prisms are made, should possess uniform density, and be free from all defects arising from irregularities in the annealing process. To detect these the glass should be carefully examined by polarized light previous to being ground and polished; and by this agent the slightest defects are made appreciable.

So also glass vessels employed for domestic purposes may be advantageously tested by the same agent. The facility with which tumblers, &c. crack, sometimes spontaneously, at other times while in the hands of the glass-cutter, or when warm water is poured into them, or when exposed to a slight hlow, depends on some imperfection in the annealing process. Hence, also, the reason why run glass (that is, glass made without paying the duty) is very apt to crack; for owing to the rapidity with which all the stages of its manufacture have been hurried on, it is not well made, and sufficient time has not heen allowed for the annealing process.

It is probable, also, that manufacturers, or rather the mounters of electrical machines, might beneficially avail themselves of polarized light in the selection of glass cylinders and plates. Recently made cylinders, when mounted, will sometimes crack, or fly, as it is termed, without any obvious agency, owing, I presume, to some defect in the annealing process, which, perhaps, might have been previously discoveied by means of polarized light.

An argument in favour of the vegetable origin of the diamond has been founded by Dr. Brewster, on the phenomena presented by this substance, when examined by means of polarized light. It is well known that various opinions have been held by different writers on the mode of formation of this mineral. All of them, however, may be included under two divisions: those which assume the diamond to be the direct produce of heat on carbonic acid or carbon, and those which ascribe it to the slow decomposition of plants. Dr. Brewster, who adopts the latter notion, met with a diamond which contained a globule of air, while the surrounding substance of the diamond had a polarizing (doubly refracting) structure, displayed by four sectors of polarized light encircling the globule. He, therefore, inferred that this air bubble had been heated, and by expansion had produced pressure on the surrounding parts of the diamond, and thereby communicated to them a polarizing structure. Now for this to have happened, the diamond must have been soft and susceptible of compression. But as various circumstances contribute to prove that this softness was the effect of neither solvents nor heat, he concluded that the diamond must have been formed, like amber, by the consolidation of vegetable matter, which gradually acquired a crystalline form by the influence of time and the slow action of corpuscular forces.

Starch grains have a laminated texture, and possess a doubly

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refractive power. They are composed of concentric layers of amylaceous matter. On some part of the surface of each grain is a circular spot, called the hilum. This appears to be an aperture or transverse section of the tube or passage leading into the interior of the grain, and by which the amylaceous matter, forming the internal laminae, was conveyed. On examining the grains by the polarizing microscope, unequivocal evidence of their doubly refractive power may be obtained. At least I have found this to be the case in all the starches which I have yet examined, viz., tous les mois, potato-starch, West Indian arrow-root, sago-meal, Tahiti arrow-root (obtained from a species of Tacca), tapioca-meal, East Indian arrow-root (Curcuma angustifolia), wheat-starch, Portland arro\v-root(Arum maculatum), and rice-starch. The larger grained starches form splendid objects for the polarizing microscope; tous les mois being the largest may be taken as the type of the others. It presents a black cross, the arms of which meet at the hilum.

On rotating the analyzer the black cross disappears, and at 90° is replaced by a white cross; another, but much fainter black cross, being perceived between the arms of the white cross. Hitherto, however, no colour is perceptible. But if a thin plate

TiesMois viewa t,;, ae of selenite be interposed bePolarizing Microscope, A . represents the tween the starch grains and the

appearance when the planes of polarization --.I,,-:-.. „,„-, cnlpnrliH anrl of the polarizer anrt analyzer are at right polarizer, most Splendid and anglesto each other; .B.when they coincide gorgeOUS Colours make their

appearance. The arms of the cross acquire the colour which the selenite plate yields in polarized light. The four spaces between the arms also appear coloured; but their tint is different to that of the cross. The colours of the first and the third spaces are identical, but different to those of the second and fourth, both of which have the same tint. At the point where the colours of the arms and of the interspaces meet, the tints blend. All the colours change by revolving the analyzer; and become complementary at every 90°.

The appearances presented by potato-starch are similar to those of tous les mois. Several other starches (as West Indian arrow-root, sago-meal, Tahiti arrow-root, tapioca-meal, and East Indian arrow-root) present black and white crosses, and, when a selenite plate is used, also colours; but in proportion as the grains are small, are their appearances less distinct. I have not hitherto detected the black and white crosses in wheat-starch, Portland arrow-root, and rice-starch. Their double refractive power, however, is proved by the change they effect in the colour yielded by a plate of selenite.

A great variety of animal structures possess a doubly refracting or depolarizing structure, as a quill cut and laid out flat on glass, the cornea of a sheep's eye, a piece of bladder, gold-beaters' skin, human hair, a slice of a toe or finger nail, sections of bones, of teeth, &c. The crystalline lenses of animals also possess this property in a high degree, owing to their central portion being denser and firmer than the external portion. If the lens of a cod-fish be placed in a glass trough of oil or Canada balsam, it presents twelve luminous sectors separated from each other by a black cross. Even living animals present double refracting properties. The aquatic larfseof a gnat, commonly called skeleton larva, form a very amusing exhibition. They are to be placed in water in a very narrow water-trough. In certain positions, they give no evidence of double refraction, but in others, and especially when they are exerting much muscular energy, they possess it in a very marked degree.

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LECTURE III.

DOUBLY REFRACTIVE AND OTHER ALLIED PROPERTIES OF CRYSTALS.

In my last lecture I explained the nature and cause of double refraction; and I now proceed to examine the double refractive property of crystals, and to show how this is connected with, or related to, other properties of crystalline substances.

1. Double Refraction of Crystals.—Every transparent crystalline body refracts the rays of light which are incident on it at oblique angles; and the degree of its refractive power depends on two circumstances; viz., the angle of incidence, and the nature of the crystalline substance. In these respects crystals agree with all other transparent media.

But a very large number of crystals possess the property of double refraction; and they are, therefore, called doubly refracting crystals, to distinguish them from others which have not this property, and which are denominated singly refracting crystals.

The double refraction of some crystals is immediately manifested by the production of duplicate images; either through two parallel surfaces, as Iceland spar, or through two surfaces which are more or lees inclined on each other. Thus to observe the double refraction of a crystal of quartz, it is necessary to look through a pyramidal and lateral plane at the same time. By this contrivance the surface of emersion is inclined to that of admission, which causes the two pencils to emerge at different inclinations, and so become further separated as they proceed.

Many crystals, however, possess the property of double refraction in so feeble a degree that it is impossible to see, under ordinary circumstances, two images; and in such cases we are constrained to employ the polariscope to detect this property.

In every doubly refracting crystal there are one or more positions or directions in which the two images become superposed;

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