Celestis, and in the Philosophical Transactions of the Royal Society. The principal services he rendered to astronomy were his prolegomena on the history of this science, and his catalogue of the fixed stars visible in our climates, which was more complete than any other then known.

Halley, who was born in 1656, and died in 1742, was profoundly skilled in geometry: but led by the prevalence of his inclination for astronomy, he enriched this science with a great number of observations and researches, which were so much the more valuable and accurate, as they had always geometry for their guide. Almost at his entrance on his career he undertook a long voyage, to make a catalogue of the stars in the southern hemisphere. As the ancients knew little more than the northern half of the globe, and those of the moderns, who had visited the southern, were tempted by prospects with which astronomy had no concern, the stars of the south, particularly those near the pole, remained either wholly unknown, or inaccurately marked on the celestial globe. To supply this desideratum, this deficient or imperfect part in the catalogues of Ptolemy and Tycho, and to make observations corresponding with those of Hevelius and Flamsteed in Europe, Halley repaired in 1676 to the island of St. Helena, the southernmost of the english possessions, lying in the latitude of sixteen degrees south, and there completely executed his design. The catalogue of the southern stars, drawn up from his observations, comprises the description of a considerable continent in the vast regions of astronomy. Halley reaped many

other

other observations from his voyage, particularly that of the transit of Mercury over the Sun's disk, which occurred on the 3d of november, 1677. This was the fourth time the phenomenon had been seen since the invention of telescopes, before which it was out of the question.

Halley was acquainted, either psrsonally or by letter, with all the astronomers of Europe. In 1679 he paid a visit to Hevelius at Dantzic; and the following year he set off on a tour to France and Italy. Being half way between Calais and Paris he perceived for the first time the famous comet of 1680, so terrible in the eyes of the vulgar on account of it's brightness and magnitude. This suggested to him the idea of writing a short treatise on comets, which will be mentioned in it's proper place.

I shall add by the way, that this same comet produced Bayle's celebrated work, entitled Thoughts on the Comet; in which that great philosopher combats with all the force of argument and reason the superstitious errours, that still existed respecting the cause and effects of the appearance of comets.

At every step a science makes, the arts connected with it, particularly those which are useful to society, are proportionally enlarged. Navigation and dialling, therefore, could not but feel the auspicious influence of the general movement, which then took place in astronomy.

By confining the use of plain charts to small extents of the terraqueous globe, the inconvenience attending them, which was that of expressing by equal lines the degrees of two parallel circles terminating the chart

on

CHAP. VI.

Progress of Optics *.

SOME writers, who have never invented any thing themselves, but find every thing in the ancients after it has been invented, ascribe to these the principal modern discoveries in optics, and the construction of the instruments dependent on them. I am willing to believe, that in this they are sincere, and not prompted by any sentiment resembling that mean envy, which is always exalting the dead at the expense of the living. But here their endeavours are in vain. We see by the oldest book on optics existing, which is commonly ascribed to Euclid, that in this branch of mathematics the ancients had only general and vague notions, some of which were even erroneous. For instance, they knew, that light is propagated in a direct line, when it meets with no obstacle in it's way; and that, when it falls on a well polished plane surface, it is reflected at an angle equal to the angle of it's incidence: but they were ignorant of the law according to which an opake body is illumined in proportion as it is more or less near the luminous body; they deceived themselves

In the general term optics are here comprised, as usual, optics. properly so called, or the science of the direct transmission of light; catoptrics, or the science of reflected light; and dioptrics, or the science of refracted light.

in making the apparent magnitude of an object depend solely on the angle under which it is viewed; they were mistaken in saying, that the place of the image formed by reflected rays is placed at their ine tersection with the perpendicular drawn from the ob ject to the reflecting surface; finally, even in the time of Ptolemy, they were acquainted only with the general phenomena of the refraction of light, and never suspected, that, when a ray passes out of one medium into another, there exists a relation between the two directions of this ray, which is subjected to an invariable law. It is certain, that optics did not begin to acquire any progress, and form a real body of science, till near the end of the fifteenth century.

One of the first who prepared or commenced this progress was the celebrated John Baptista Porta, a neapolitan gentleman, the inventor of the camera obscura, who was born in 1445, and died in 1515. In his work entitled Magia naturalis he says, if we make a small hole in the window shutter of a room, from which the light is every where else completely excluded, we shall see the external objects depict themselves on the wall, or on a piece of paper, in their natural colours: and he adds, that, by placing a small convex lens at the aperture, the objects will appear so distinct, as to be recognised instantaneously. From these assertions, verified by experience, there was but one step to the explanation of the mechanism of vision. This Porta did not completely make: be only remarked, that the bottom of the eye might be considered as a camera obscura, without pursuing

or entering into any explanation of this just and happy idea.

Maurolicus treated on the general theory of optics in two works; one entitled Theoremata Lucis & Umbra; the other, Diaphanorum Partes seu Libri tres. These works contain several curious researches on the measure and comparison of the effects of light; on the different degrees of brightness, which an opake object receives from a luminous body, according as it is more or less distant; &c.

If Maurolicus have not always hit the truth, at least he has pointed out the way to his successors, and spared them many futile attempts. He has very justly explained a well known phenomenon, on which the ancients, and in particular Aristotle, had given us nothing but reveries: this is, that the rays of the Sun, when they pass through a small hole of any shape, a triangular one for example, always form a luminous circle on a piece of paper parallel to the hole, and placed at some distance. In the first place Maurolicus observed, that, when the paper is placed very near to the hole, this must be depicted on it in it's actual figure; but as the paper is removed farther off, the similitude gradually disappears, till at length the image becomes circular. In fact, each point of the hole may be considered as the vertex of two opposite cones, one of which has for it's base the Sun, the other a luminous circle cast on the paper by the decussation of the rays at the vertex; and as the number of points in the hole is infinite, so is that of the cones. Now the circles formed on the paper by

the