Among the many works published on the science of Optics, may be mentioned the following: adapted to persons of different capacities and attainments in science.

"Scientific Dialogues," vol. v. being part of the series already noticed, intended for those who have a taste for science without having entered much into the elements of it.

"A Short System of Optics, principally designed for the Use of Under-graduates in the University of Dublin, by the Rev. John Stack." This little work, though rarely to be met with in England, deserves notice on account of its excellence as a compendious introduction to the science of Optics. The reader will be required to know the elements of common geometry, as the demonstrations of Mr. Stack, though very easy, are purely geometrical.

"The Elements of Optics, by James Wood, B. D," make part of the Cambridge course of Mathematics and Natural Philosophy. In addition to the same subjects treated on by Mr. Stack, it enters at large on the Aberrations, produced by the unequal refrangibility of different kinds of rays, and by the spherical form of reflecting and refracting surfaces: a short section is devoted to the doctrine of Caustics. This is more abstruse, and requires a larger share of mathematical knowledge than the preceding, or than the next which follows.

"A Treatise of Optics, containing Elements of the Science, in two books, by Joseph Harris, Esq. late His Majesty's Assay-Master of the Mint," 4to.

Those persons, who have not leisure, nor perhaps a sufficient share of mathematical knowledge to go through the whole of "Dr. Smith's Complete System," hereafter to be mentioned, will find Mr. Harris's treatise a valuable introduction to the science of Optics. It is, indeed, in general, but a compilation; the materials, however, that compose it, are arranged with judgment and perspicuity; and the demonstrations are neat and easily comprehended, by those students who

have laid a proper foundation in the elements of mathematics. In the first book, the elementary part of optics is explained: after the necessary definitions, the author demonstrates the various laws of refraction and reflexion:-he then illustrates the rationale and effect of lenses of different figures; and after having laid down the theory of the reflexion and refraction of light, Mr. Harris, in the second book, explains the subject of vision.

"A New and Compendious System of Optics, by Benjamin Martin," is a popular treatise, illustrated with experiments and examples; of the latter, many are worked by common arithmetic. This, like several other of Mr. Martin's works, abounds with typographical errors.

"A Complete System of Optics, in four books, by Robert Smith, LL. D. Professor of Astronomy and Experimental Philosophy at Cambridge," 2 vols. 4to.

The first part of this elaborate work, is designed for the use of those who would know something of Optics, but who want the preparatory learning that is necessary for a thorough acquaintance with the subject. With this view the author has avoided all geometrical demonstrations, and substituted the more entertaining sort of proof, drawn from experiments that may be repeated with little trouble or apparatus. By this means any one with moderate application, may make himself master of no inconsiderable part of the doctrine of optics. The second book is a complete mathematical treatise of the science; and will require, in the reader, a large portion of geometrical and algebraical knowledge. In the third book is given a description of a complete set of optical instruments, with explanations of the various uses to which they may be applied, in Astronomy, Geography, Navigation, Levelling, &c. An history of the telescopic discoveries in the heavens, is the subject of the fourth book, which modern discoveries have, of course, rendered imperfect.

"The elementary parts" of Dr. Smith's Optics, were published by Dr. Kipling, in 1778, who added, in the form

of notes, some explanatory propositions from other authors, chiefly from Dr. Barrow and Descartes.

To the elementary works on the science of Optics generally, may be added the following on a particular branch of it.

"Of Microscopes, and the discoveries made thereby; illustrated with many plates, by Henry Baker, F. R.S." This work, which consists of two volumes octavo, contains much useful knowledge exhibited in a simple and perspicuous method, for the sake of persons who have not had the advantage of a learned education. The reflections, which Mr. Baker draws from the various parts of these volumes, all tend to impress the reader with just ideas of the wisdom, power, and goodness of the great Creator. "The minute living animals, exhibited in the second part of this work, will excite a considerate mind to admire in how small a compass life can be contained, what various organs it can actuate, and by what different means it can subsist. They will also shew that the hand which made them, is not confined to size or form: and that it has not been wanting to bestow on creatures almost invisible, and seemingly inconsiderable, every member and faculty convenient to their happiness." These volumes, which were long extremely popular, and went through many editions, have given way, in a great measure, to a work of the same kind, in 4to. by Mr. Adams. And within the last year or two, the author of "the Scientific Dialogues," has published "Dialogues on the Microscope," in two small volumes, which include what is important and interesting in Baker, and the discoveries of all the latest writers on the subject. This work contains an account of the principles of vision, and of the construction and management of the most improved and generally useful microscopes, with their application to the discoveries made by them in the different kingdoms of the natural world. They are accompanied with eight highly finished plates, by Mr. Porter,

state, produces no sensible effect; but as soon as the equili brium is disturbed, and any body, or surface of a body becomes possessed of more or less than its natural share, it exhibits the phenomena of attraction and repulsion, and other remarkable effects are produced according to the quantity of excitation.

The equilibrium could never be disturbed, or if it were, it would be immediately restored, if the bodies in, or on which it was disturbed, admitted to it a free passage, either along their surfaces, or through their pores. As this is not the case with some bodies, whenever any of these has acquired an additional quantity of electric matter, and is at the same time every where surrounded with bodies through which it cannot pass, it must remain overcharged: or if it has lost a part of its natural quantity, it must in like circumstances remain exhausted.

All substances are divided by electricians into two classes called electrics and non-electrics. Electrics do not suffer the electric matter to pass readily over their surfaces, and bence they obtain the name of non-conductors. Non-electrics do permit the electric matter to pass readily over their surfaces, and on that account they have been denominated conductors.

Glass, resin, sealing-wax, sulphur, bees-wax, and baked wood, are the most perfect non-conductors among solids, and oils, and dry air among fluids. But heat and moisture render all substances conductors.

According to the theory of Franklin, which seems best adapted to the explanation of facts, when a body has more than its natural quantity of this fluid, it is said to be positively electrified, and the electricity which it contains is called positive electricity; and when it has less than its natural share, it is said to be negatively electrified. When a conductor is so surrounded with non-conductors, that the electric fluid cannot pass from it into the earth, it is said to be insulated, such is the case of a piece of metal supported upon a glass pillar.

The electric fluid is generally brought into action by friction, though there are other means of disturbing the equilibrium. The chief electric appearances are the attraction and

repulsion of light bodies; fiery sparks attended with a snapping noise; pencils of light proceeding from, or entering into an electrified body upon the approach of other substances.

If a glass tube two or three feet in length, and an inch or more in diameter, be briskly drawn through the hand, or rubbed with a piece of dry and warm silk, the effect of that friction is, that the electric matter leaves the hand, and passes upon the glass, where it will remain; for as the glass, and the dry air which surrounds it, are non-conductors of electricity, this redundancy of the electric matter cannot flow away; but if any conducting substances, as the finger, or a piece of metal, be presented to the glass, the electric fluid will pass from that part into them, attended with a crackling noise. The tube thus excited will attract any light substances, as morsels of paper, &c.; which will, when brought near, spring to it; and after a few seconds, when they seem to have obtained a certain portion of the electricity of the glass, they will be repelled from it. This, and all electrical experiments are best seen in a darkened room.

Although the electric fluid may be excited with the hand on glass tubes, or on sticks of sealing-wax rubbed briskly with flannel, or on the sleeves of a man's coat; yet the quantities so obtained are very small in comparison of what may be collected by other means, as an electrical machine, and may be wanted for experiments on a large scale.

Fig. 1. Plate IV. represents an electrical machine; the construction is evident, almost at first sight. EF is a glass cylinder turned by the handle I, and the whole frame is fastened to a strong table by iron clamps. K is a rubber made of silk, or leather stuffed with horse-hair, and L is a silk flap fastened to it, and covering part of the cylinder, to prevent the dispersion and escape of the electric fluid. The rubber is fastened to a spring, which proceeds from a socket cemented on the top of the glass pillar M. There is another strong glass pillar N, on which is fixed an hollow metallic cylinder OP, called a prime conductor, to one end of which is attached a row of pointed wires