I hope I fhall not be thought hyperbolical in recommending this book, if I add, that it will be of great ufe to the man of regular and complete learning; to him who, from his juvenile years, hath applied a part of every day to the cultivation of his mind. From my refpect to the dignity of fuch a character, I only prefume to offer it to him as a literary common-place book. His mafterly knowledge, and the alphabetical order of the work warrant the appellation. Let me obferve, however, that the contents of a common-place-book, which is the depofitary of intelligence to the learned and the liberal, are most worthy of remembrance. He must be a very fupercilious scholar, or a very conceited pedant, perhaps of capacious, but certainly of dry and abstract memory, who despises a comprehensive view of the celebrated nations of antiquity, whose inftitutions, cuftoms, and manners, are here compendiously and accurately related-of the Egyptians, Carthaginians, Cretans, Peruans, Athenians, and Lacedæmonians.

As to those whom fortune has deprived of the opportunities of a good education; and who have not had fufficient fortitude to encounter the perfevering labour of literature, without which we cannot earn its indeprivable and sublime enjoyments, (I am addreffing myself to thofe who read for amusement) I beg leave trongly to recommend this book to their perufal, as it will afford them rational amufement; as it unites the furprifing incidents and characters of romance with the useful inform ation of historical truth; and while it gives a lively pleasure to the imagination, enlarges the knowledge of human nature. There is a clafs of readers who are only converfant with thofe books which give a frivolous detail of European amours, or exhibit a barbarous glare of Afiatic fplendor. I fhould be happy to perfuade them to correct their vitiated tafte, to afpire to the pleasure of intellectual beings, to refolve to join the utile with the dulce; and to be at once entertained and improved. Both thefe ends may be attained by the judicious choice, and attentive perufal of travels and hiftory. And here, if they are fond of the marvellous, their fancy will be warmly actuated by many prodigies in the phyfical as well as in the moral world. They will be interested in objects worthy of their admiration objects lefs gorgeous, but far more noble and more important to man than the machinery and the genii of an oriental fabulift. For the talifman of the Eaft, let them be entertained with heroic virtue, which has wrought many miracles. For an enchanted caftle, let them fubftitute a manfion finitely more auguft and awful, the facred cottage of an old Roman dictator. Let their dwarfs be reprefented by our modern petit-maitres: and they will certainly not be lofers, if they exchange their giants for an Annibal, a Timoleon, and an Epaminondas.'

in

We fhall conclude with obferving, that the verfion is executed with that freedom and fpirit, that ease and energy,

which convey the fentiments with equal precifion and elegance, and give the work all the genuine air of an original compofition.

VIII. A Treatife of Optics: containing Elements of the Science. In Two Books. By Jofeph Harris, Ejq. 4to.

White.

145. boards.

Notwithstanding this book is but lately published, it seems

to have been printed off about thirty years fince, as may be gathered from the following Advertisement prefixed to the work, which it is neceffary to extract, becaufe it contains an hiftorical account of the work.

The author of the following work, in the year 1742, first propofed printing a Treatife upon Microfcopes only. The plan laid down was entirely new; and to render it more complete, a prefatory difcourfe was intended, to introduce fuch general principles as were neceffary for understanding the fubject fcientifically. This difcourfe increased fo faft, under the author's pen, that it it was foon thought advifeable to enlarge the original defign, into a general Treatife of Optics. Two books, containing the elementary parts of this fcience, being finished with all convenient expedition, were immediately printed offboth plates and letter-prefs-with uncommon care and correctnefs. And upon this foundation, it was propofed to explain the Theory and Mechanifm of Optical Inftruments, in a third, book, under the general head of Telescopes und Microscopes. A talk of this kind, required a multitude of accurate experiments; and perhaps nobody could be better qualified for making or reafoning upon them, than Mr. Harris. But, while this bufinefs was forwarding with unufual celerity, our author was called from the purfuit, by an extraordinary demand of duty in his majesty's mint and from that time, a variety of avocations, for the public fervice, with a feries of bad health, the confequence of laborious application and ftudy, prevented any regular renewal of the fubject for feveral years. Mr. Harris however, did not lofe fight of his favourite object, but employed moft opportunities for the advancement of it; till at length, finding himself once more at leifure, he refumed his application to this work with an earnestnefs that probably hastened his diffolution for, in the actual profecution of feveral interefting experiments he was taken fick, and, unfortunately for the public, expired 26th September 1764.

After Mr. Harris's deceafe, a numerous collection of manufcripts were fubmitted to the confideration of fome gentlemen, well acquainted with the fcience of optics, in hopes of completing to valuable a work. But it appeared, upon examination, that though there feemed to be ample materials for the purpose,

and

and many of them prepared for the prefs; yet the greater part, having never been digefted, was incapable of any arrangement, without affiftance of the author-no longer to be had-or taking up the fubject de novo, and purfuing it through an almost infinite variety of experiments-a labour, neither to be expected or defired of a ftranger!

• Under these circumftances, the whole Treatife has long lain dormant, against the importunities of many friends, who have frequently recommended to publish this elementary part alone. They have repeatedly alledged-that the elements of science are always the fame-that these elements, being quite diftin&t, are complete in themselves-that very little has been written on the fubject and that the following fheets, containing much new matter, would be a valuable acquifition to the science. Such have been the reafons advanced for this publication, by gentlemen whose names might do honour to any book: and they have at last prevailed over every fcruple of the proprietor. But as nothing here is intended to bias the favour of the public, the true history of this work is fairly related, and the performance fubmitted, upon its own merit, to the candid judgment of the impartial reader.'

By this account it feems that this work was compofed foon after the publication of Dr. Smith's treatife on the same subject, to which it bears a near refemblance in the manner of treating and form of printing it. With refpect to the execution of the plates, which are numerous, we think them fuperior to any of the kind we remember to have feen. As this work was originally intended as an introduction to optical inftruments, it is adapted, in its contents and execution, to that particular purpofe; though many other parts of the fubje&t are very well treated and added in their proper places, to form the whole into a general Treatife on Optics. Though we do not meet with many new discoveries on the subject, yet the author has fhewn himself not only a good writer but an excellent optician, by the judicious arrangement of the parts, and the masterly manner of treating them, befides the many curious anecdotes and philofophical reflections and difquifitions added at the bottom of the pages by way of notes to the general text of the work, which is more mathematical.

This publication is divided into two books, and each of thefe into many different fections and chapters. In the former is given the elementary part of optics, independent of the ftructure and confideration of the eye; in the other, we have thofe parts which require the explanation, &c. of the eye, and manner of vifion. In the first book the author delivers an account of the known chief properties of light, and of fome general terms used in the fcience he next treats on the re

flection

Bection and refraction of the rays of light at plane and fpherical furfaces, with the progrefs of the rays through them, and the images formed by them: we have then the application of thefe principles to lenfes of glafs, &c. treated in a very full and geometrical manner, especially the abberrations from the geometrical focus caufed by the figure of the glass, and by the different refrangibility of the rays of light.

In the fecond book he applies the foregoing principles to the eye as a kind of compound lens, and first gives a fhort, but very clear defcription of the feveral coats and humours of the eye, with an account of their proper fituations, dimenfions, and ufes; the whole illuftrated by a proper figure, from which a very clear idea of the use of the feveral parts of the eye, in vifion, is obtained. In a propofition, and its corollaries, he then applies the theory of lenfes, before laid down in the first book, to an eye of the dimenfions and refractive powers here mentioned, as found from experiments, in order to find the place of the image of an object formed by the humours of the eye.

Among many curious obfervations concerning the manner and means of vifion, he gives the following curious extract from a MS. of Sir Ifaac Newton, communicated to him by Mr. Jones.

Light feldom ftrikes upon the parts of grofs bodies, (as may be feen in its paffing through them;) its reflection and refraction is made by the diverfity of æthers; and therefore its effect upon the retina can only be to make this vibrate: which motion then must be either carried in the optic nerves to the fenforium, or produce other motions that are carried thither. Not the latter, for water is too grofs for fuch fubtile impreffions; and as for animal fpirits, though I tied a piece of the optic nerve at one end, and warmed it in the middle, to fee if any airy fubftance by that means would disclose itself in bubbles at the other end, I could not spy the leaft bubble: a little moisture only, and the marrow itfelf fqueezed out. And indeed they that know how difficultly air enters fmall pores of bodies, have reafon to fufpect that an airy body, though much finer than air, can pervade and without violence (as it ought to do) the fmall pores of the brain and nerves, I should fay of water; becaufe thofe pores are filled with water and if it could, it would be too fubtile to be imprifoned by the dura mater and fkull, and might pafs for æther. However, what need of fuch fpirits Much motion is ever loft by communication, efpecially betwixt bodies of different conflitutions. And therefore it can no way be conveyed to the fenforium fo entirely, as by the ether itself. Nay, granting me, but that there are pipes filled with a pure tranfparent liquor paffing from the eye to the fenforium, and the vibrating motion

of

of the æther will of neceffity run along thither. For nothing in terrupts that motion but reflecting furfaces; and therefore alfo that motion cannot ftray through the reflecting furfaces of the pipe, but must run along (like a found in a trunk) entire to the fenforium. And that vifion thus made, is very conformable to the fenfe of hearing, which is made by like vibrations.'

In fome following pages feveral other extracts are given from the fame MS. Among many other reflections on our manner of acquiring ideas by vifion, our author fays,

As all our fenfations are fome how caufed by impreffions from external bodies, fo particular fenfations never fail to excite in us the ideas of particular bodies; and these ideas, or fomething at least in the compofition of them, we undoubtedly acquire by experience. How elfe could a particular found produce the compound idea of a bell, a drum, a man, an ac-, quaintance? And language is a furprising inftance of the great ftrength and quickness of our retentive faculties, in affociating together a vast variety of ideas, which before were learned and treafured up, as the proper fignifications of fuch particular words or founds. In like manner, different fmells and tastes are accompanied with ideas, which before had been treasured up in the memory. Our fenfations themfelves are probably coeval with us, and perhaps they differ but little all our lives, except in degree of quickness or perfection: it is the ideas that accompany them, which are acquired; and thefe for the most part are got fo early, that we forget our having ever learned, as it may not improperly be called, to fee and to hear, &c. And yet fo true it is, that we have as it were learned feeing and hearing, &c. that an adult perfon, who had been blind or deaf from his infancy, would not be able immediately to make any great ufe of his new acquired fenfe, till he had time and opportunities to make his proper obfervations, as others had done in their childhood.'

Inftances of this may be feen in the Philofophical Tranfactions, No. 402; and in the Tatler, No. 55; and elsewhere. In treating of motion as evinced to us by vifion, the author has these curious remarks concerning the greatest and least vifible motions.

Apparent motion, or the motion of the images of a moving object upon the retina, must have a certain limited degree of velocity to become perceptible; that is, the fpace defcribed upon the retina in a given time, must be neither lefs than fome given fpace, nor greater than fome other determinate fpace.

If the motion of the fun's fhade, at the distance of five feet from the gnomon that cafts it, be obferved at about a foot diftance from ths fhade, when the fun changes his azimuth about ten degrees in an hour; the apparent motion of the fhade