TRANSPARENCY is that quality of certain substances or media by which rays of light are allowed to pass freely through them. It is doubtful whether any substance exists which is perfectly transparent ; for even water and air stop more or less of the light passing through them when the length of its path is very great. It is, however, exceed ingly difficult in such cases to say whether the observed stoppage be due to the pure substance, or to foreign bodies present in proportions otherwise perhaps inappreciable. On the other hand we have reason to believe that all bodies possess the property of transparency in a certain degree. Thus many metals have been obtained by mechanical or chemical means in such a state of thinness as to transmit a certain amount of light ; for example, gold, which in the state of gold-leaf transmits a greenish light.
There are two distinct obstacles to transparency, one a defect of homogeneity, whereby a portion of the light in its onward progress is continually reflected in another direction, at the surface of separation of adjacent portions of the body having different refractive powers; the other the power which a very great number of substances possess of absorbing light. [Anscierrios OF LIGHT.] A good example of the former is afforded by snow, which in sufficient thickness prevents the transmission of light, but simply in consequence of its reflecting the light backwards at the various surfaces of the icy crystals ; and accordingly snow is brilliantly white by reflection, whereas the single reflection from a sheet of water or ice is comparatively feeble. Other examples are afforded by a mixture of water and oil, or water and bisulphide of carbon, shaken up together ; or better still, an alcoholic solution of bisulphide of carbon precipitated by the addition of water. In these cases mixtures are obtained which, from the multiplied reflections, have a milky appearance, and in thickness stop transmitted light, though the mixed fluids (bisulphide of carbon and watery alcohol in the last example) are separately transparent. The transparency of white paper is greatly increased by oiling the paper, the reason of which is that the quantity of light reflected at the com mon surface of the fibres and oil, which do not very greatly differ in refractive power, is very much less than that reflected at the common surface of the fibres and air. The mineral hydrophane derives its name from becoming more transparent after being placed in water, which is in consequence of its imbibing water in the pores with which it is filled.
Examples of the second obstacle to transparency are sufficiently familiar, as may be gathered from the article on ABSORPTION. Thus the common blue glass cuts off a large quantity of the light incident upon it, and when in tolerable thickness may for most purposes be regarded as opaque. Most commonly both obstacles to transparency exist together, as in the case of wood, cork, brick, dyed cloths, &c.
That homogeneity should be one requisite for transparency, follows from the existence of reflection at the common surface of media of unequal refractive power ; and therefore in considering the cause of transparency it will be sufficient to confine our attention to homo geneous media. In such media transparency is to be contrasted with the power of absorbing light ; the former in the absence of the latter.
The speculations at one time entertained respecting the cause of absorption on the supposition that light consists in particles darted forth by the luminous body can now only be matter of history; we shall confine ourselves to a consideration of the probable cause of absorption, on the supposition that light consists in the tremors of an elastic medium.
In the case of gaseous media especially, the rate of absorption of light passing through them changes, in many instances, in a very remarkable manner with the refrangibility of the light. Hence the spectrum of white light subjected to prismatio absorption on the part of such media, presents fluctuations of intensity, simulating more or less completely effects due to interference. Accordingly attempts have been made to refer absorption to ordinary interference. But such explanations labour under one fatal defect ; they suppose the annihila tion of 'as viva. The effect of ordinary interference is not to destroy light, but merely to alter the distribution of illumination. Thus, when the light of a rather distant candle is reflected from a thin plate of mica bent into a cylindrical form, and the linear image of the flame is analysed by a prism, it is true that dark bands are seen which remind one of the bands produced by the absorption of light by the vapour of iodine, and were applied by the Baron von Wrcde to tho explanation of the latter and of absorption in general. (Poggendorff's Annalen, vol. xxxiii. (1834), p. 353, and Taylor's ' Scientific Memoirs,' vol. i., p. 477.) But there is this difference between the two cases, that in the experiment with mica, the light which is defective in the reflected beam is found in excess in the transmitted beam, which would yield a spectrum having bands com plementary to those of the former ; whereas in the case of absorption the missing light actually disappears as such. It is not however annihilated; an effect is produced on the medium, whether it be that the temperature is raised, or that chemical changes are produced, or that the medium is made to omit light of a different kind, as in the phenomena of phosphorescence .and fluorescence. LFLvonEsceicce.] These phenomena, and especially perhaps the last, indicate that the molecules of the medium are thrown into a state of agitation ; and thus we are led to suppose that the uudulationa of the luminiferous ether are spent in producing agitations among the ultimate molecules of the absorbing body, the consideration of which therefore must form an part of a complete explanation of absorption. That the period of the incident undulations should play such an important part in the phenomena, may be illustrated to a certain extent by considering the effect of a series of alight pushes, periodic lly applied to a body capable of swinging as a pendulum, which will throw it into a state of con siderable vibration, provided the period of the pushes nearly agrees with that of the natural vibrations of the body. If the cause of absorption be that just explained, we must attribute transparency to the existence of such a coustitution in the body, that the other, or a portion of it at Jesuit, can in its undulations glide freely among the molecules of the body, without throwing them into a state of agitation.