BOOK I

In this part of his Essay the author has grouped together a variety of phenomena, some of which are meteorological (in the modern sense of that word), and belong therefore to his class of Sublimia, while others are astronomical, and would be properly placed among his Caelestia. They all have reference to light in some form, and doubtless for that reason were considered as a series. Seneca, largely swayed by the opinions expressed in Aristotle’s _Meteorologica_, agrees with that philosopher in the belief that the earth gives forth various kinds of exhalations, among which some contain the seeds of fire. He thought that high up in the air, among dry and hot elements, these fires may be kindled by the sun’s rays, and further, that when the atmosphere becomes violently disturbed its friction may give rise to fires (9, 10, 39).

With these ideas, which he held as established truths, it is easy to understand that he should have regarded as extremely foolish the notion that any of the lights which move rapidly across the sky are of celestial origin. Had such been their source, he felt sure that by this time there would have been none left in the firmament; yet although no night passes when some of them may not be seen, each star in the sky is found to maintain its place and its size. Hence he confidently concluded that the meteors, which are seen at night, and sometimes even by day, have their birth far below the stars, and are soon extinguished in their course because they have no solid and abiding resting-place. Single aerolites and even showers of stones had been recorded in Roman literature as having fallen from heaven, but it had not yet occurred to any observer to connect them with the shooting stars which gleam across the nocturnal sky, and are now recognised to be due to meteorites of different sizes, entering our atmosphere with planetary velocity, there breaking up with varying luminosity, and remaining visible for shorter or longer intervals of time.

The author appears to have regarded as akin to these meteors the star-like balls of light, which in stormy weather are sometimes seen on the masts of vessels at sea, and which before his time had been observed on the points of the spears of an army in the field. This luminous appearance, regarded by the Romans as a sign of the friendly presence of Castor and Pollux, is entirely atmospheric, and has no connection with shooting stars. It is now known as St. Elmo’s Fire, and has been shown to be a gentle continuous electric discharge from the earth towards a cloud.

Seneca next describes in some detail a series of optical appearances connected with the sun and moon. Until the laws of the reflection and refraction of light had been discovered, it was obviously impossible to account for these phenomena. There is, therefore, much interest in following the lines of thought by which the old philosophers attempted to explain them. Seneca clearly perceived that the halos and coronae seen round the sun and moon in certain states of the atmosphere do not belong to these luminaries, but to our own air, and may furnish indications of coming weather. He remarks shrewdly enough that appearances akin to those seen in the sky may sometimes be observed in the thick moist air of a bathroom. But when he confidently proceeds to explain the meteorological phenomena he betakes himself to analogy, as he is so fond of doing. He remarks that when a stone is thrown into a pond a succession of circles is produced on the surface of the water, which continually widen from the point of impact until they lessen and disappear. In like manner he believes that when the light of the sun or moon strikes the cloudy air it produces a similar effect, for as every kind of light is round in shape, the air is thus driven into a circular form. His love of analogy generally, as in this instance, leads him far away from the truth, and prevents him from seeing the palpable flaws in his reasoning. But the apparent similarity of appearances, which are in reality entirely dissimilar, contents him with his explanations.

His discussion of the rainbow (16–33) is one of the most detailed and vivacious in the whole volume. It takes the form of a sustained argument, in which the author cites various authorities, and replies to objections brought by a supposed opponent to his thesis, which is that the rainbow is unquestionably an image of the sun received in a very moist cloud which has the shape of a round concave mirror (20, 27). He quotes with apparent approbation the opinion that in a shower of rain each falling drop is a mirror reflecting an image of the sun, and that when an observer stands directly between the sun and the shower he sees the reflections of the countless drops blended into one continuous semicircle. But as the discussion proceeds the writer denies that the cloud consists of separate rain-drops, and he maintains that even if it did they would not unite to give one unbroken image. In proof of his contention he urges the fallacious assertion that if a number of mirrors are joined together and a man is placed before them, each gives its own reflection, and thus a single man becomes multiplied into a crowd. If he had ever tried the experiment or had visited the shop of a mender of mirrors, he would have seen that the separate pieces, if strictly arranged on the same plane, reflect a single image. His imaginary antagonist asks for an explanation of the rainbow-like colours displayed by the spray from a burst water-pipe, or the splash from an oar, which are, of course, cases strictly parallel to the falling shower of rain (24). The resemblance is at once granted, but is explained away on the ground that the drops fall so quickly that they cannot form reflections of the sun, and that to produce such reflections the medium must be at rest. The objector once more strikes in with a reference to the rainbow colours to be seen in a glass rod which is placed obliquely in the path of the sun’s rays (30). These prismatic tints, as has long been known, are due to the same decomposition of white light, as in the rainbow. But Seneca claims the illustration as furnishing additional arguments in his favour. He maintains that no colour is really produced in the rod, but only a false appearance of colour, his idea being apparently that unless the colour is inherent in an object apart from direct sunlight, it is only apparent and not real. The glass, he says, tries to reproduce the sun’s image, but fails because of its unsymmetrical form, the reflections being crowded together and confused into the appearance of a single band of colour. In regard to the falling drops of rain in a shower he contends that they receive the colour but not the image of the sun, and he is led away by the false analogy of the varying tints of a peacock’s neck as the bird tosses its head (25). At one part of the discussion he affirms that the colours of the rainbow come partly from the sun and partly from the moist cloud (21). Further on, however, he agrees that they proceed from the sun, but are only apparent, for if another cloud comes across the face of the luminary they at once vanish (29). The greater diameter of the rainbow compared with that of the sun as seen by us he accounts for by the analogy of a concave mirror, which greatly enlarges the objects reflected from it. At the conclusion of the discussion he repeats his belief that the rainbow and the corona or halo have no definite material inherent in them, but are like a mirror which reveals only a deception, the mere phantoms and empty imitations of real bodies, which certainly do not exist in the mirror, and therefore cannot come out of it (41).

In Chapters XVI. and XVII. the author indulges in one of his favourite moralising episodes, suggested by the topics he has been discussing in the previous pages. He takes the existence of reflecting surfaces as his text, and from the calm surface of still water passes on to artificial mirrors, contrasting the manners and morals of early mankind, who had only pools and lakes in which to see their faces, with the luxury and vice of later ages, when the use of metals led to the invention of metallic mirrors. In this retrospect, however, he places the discovery of the use of iron before that of the other metals. The priority of bronze and the reason for it are accurately stated by Lucretius:

et prior aeris erat quam ferri cognitus usus, quo facilis magis est natura et copia maior.[110]

[110] _De Rer. Nat._ v. 1287.