incandescent substance-and the flame of a candle is such light-exhibits a continuous spectrum. The double bright line seen by Fraunhofer was due to the presence of the almost ubiquitous element, sodium, in the flame, as will be explained further on.

If we carefully weigh the results obtained by Fraunhofer, it will appear that spectroscopic analysis owes very much to his researches. It may be questioned, indeed, whether, but for his patience and perseverance, the attention of scientific men might not have been turned away, at least for many years, from a subject of inquiry which seemed when he began his labours to be rather curious than important.

Let us next inquire into the spectra given by different terrestrial sources of light.

An incandescent solid or fluid-or, to speak more correctly,* a solid or fluid glowing with intensity of heat gives a continuous spectrum. But the nature of the spectrum varies with the heat of the source. If a piece of metal, for instance, be gradually heated till it is at a white heat, only the red part of the spectrum will at first be visible; then the orange part will show,

*The term incandescent is not properly applicable to any source of light which is not actually white. Many spectroscopists indeed go farther, and say that no luminous body ought to be described as incan descent unless its spectrum extends without dark lines or gaps of any sort from the extreme limit of visibility at the red end to the extreme limit of visibility at the violet end of the solar spectrum. Without insisting on this limitation, it certainly does seem well to point out that the term incandescent is not properly applicable to solids or fluids glowing with light belonging to the red end of the spectrum, nor to vapours glowing with light of a well-marked colour.

then the yellow, and so on, until at length the whole range of the spectrum will be seen, from the extreme red to the extreme violet.

But the question will at once suggest itself at this point,-What are the limits of the spectrum? When we were considering the solar spectrum, we might have inferred that the limits of visibility are the true limits of the space to which the solar action extends. But now that we have seen a spectrum growing with the growing heat of the source of light, we are naturally led to inquire whether there are limits to this growth. And, again, the spectrum of a gradually heated body begins at the red end so far as our vision is concerned; but is that its true beginning?

To both these questions an answer has been given. It occurred to several physicists during the latter half of the last century to inquire whether the heat which undoubtedly accompanies the light forming the spectrum corresponds in reality with the light, in so far that where the light is strongest the heat is strongest, and vice versâ, while where light fails totally there heat also fails. They found that no such correspondence exists. So far as the visible spectrum is concerned, the greatest heat is not received where the spectrum is brightest, but at the red end. Sir W. Herschel, however, at the beginning of the present century, found that the maximum of heat comes from beyond the red end of the spectrum; while, yet farther beyond the red end, heat continues to be received for a distance whose extreme limit has not yet been determined.

Heat, however, is not received beyond the violet end of the spectrum, nor even completely up to the limits of visibility in that direction.

Yet we now know that in this direction, also, the limits of visibility are not to be regarded as the limits of the solar action. Besides the heat and light which are transmitted through the prism, there is another form of action whose effects are as distinct from heat and light as heat and light are from each other. We know that the solar rays, besides illuminating and heating substances on which they fall, produce changes in the appearance and constitution of many substances. To take a familiar example: the solar rays falling on the skin not only warm it, and so affect the sense of touch; not only illuminate it, and so affect the sense of sight; but tan it, an effect which is not directly cognisable by any sense that we possess,* though in

* It need hardly be said, perhaps, that the reference here to the senses is not introduced as bearing in any way on the subject of spec troscopic analysis, but as affording a distinction of a popular kind between the three forms of solar action. It is worthy of notice, however, that we have a sense by which the action of the longer light-waves corresponding to the red end and the parts beyond the red end of the spectrum is recognised by us, and another sense enabling us to recognise the action of the medium waves corresponding to the yellow part of the spectrum and in gradually diminishing degree the waves corresponding to parts up to the red end on one side and the violet end on the other side: but we have no sense enabling us to recognise directly the action of the shorter waves corresponding to parts of the spectrum beyond the violet end. Is it not conceivable that some creatures, even among terrestrial beings, may possess a sense enabling them to recognise the action of these short waves, and that such a sense may give them powers as distinct from the powers we possess in virtue of the senses of touch and of sight, as the sense of sight is distinct from the sense of touch? A man born blind may not be more incapable of conceiving the nature of the sense

directly sensible both to sense and touch. The real nature of this action is not altogether understood (which may be remarked also respecting heat and light), but the observed result is a modification of the chemical condition of the substances acted upon.

Now this particular mode of action-actinic action, as it has been called*-is not exerted most powerfully where the spectrum is brightest, but near the violet extremity-between the lines G and H. Nor is this mode of action limited to the visible spectrum; but precisely as heat falls beyond the red extremity of the

spectrum, so actinic effect is produced beyond the violet extremity.

In fig. 24 the relation between the several kinds of action is exhibited by the curves marked H, L, and ch respectively, the height of these curves vertically above

of sight and of the powers it confers upon those who possess it, than those who have all the five senses are of the powers which may be actually possessed by creatures having organs suited to appreciate the action of the shorter light-waves.

* From a Greek word signifying a ray. Surely the term is illchosen.

the spectrum indicating the relative intensity of the heat, light, and chemical activity of the portion of the spectrum immediately below. Thus, beyond the red end, the heat-curve is seen to acquire its greatest height, indicating that the maximum heating effect is exerted there. The light-curve reaches its greatest height above the part of the spectrum between D and E, -in the yellow portion, that is, of the spectrum. The heat-curve reaches the base-line close by G, while the light-curve extends somewhat beyond H. At E the actinic curve rises above the base-line, reaching its greatest height above G and H, and thence passing down to the base-line far to the right of the violet end.

It appears, then, that from the extreme heat end of the spectrum to near A there is heat only; from near A to about E we have heat and light combined; from E to G, all three forms of action-light, heat, and actinism are present.* From about G to a little beyond H, light and actinism (though very little of the former) are exerted, while from a little beyond H to the extreme actinic end of the spectrum there is actinism alone.

It may be well, however, to warn the reader against the error sometimes made, of assuming that where two forms of action are present they are separable (or can be conceived to be separated) from each other. For example, it would be incorrect to say that near D a small proportion of heat rays, and a larger proportion

It is worthy of notice how small the amount of all three forms of action becomes near the F line of the spectrum.