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with the increasing avidity with which the study of Nature is now pursued, and the great numbers, in all parts of Europe and America, who are applying themselves to it, he may be almost assured that he will not be disappointed, provided the fources of information be easily open to him.'
The person who figures first in this period is Bap:ista Porta, the inventor of that curious infirument, the Camera Obscura; and who, from the phenomena presented by it, was convinced that vision is performed by the incromiffion of something inte the eye, and not by visual rays procceding fisin it, as had been the generally received opinion before his time. The various discoveries and observations of this writer, of Maurolycus who preceded him, of Lord Bacon, of De Duminis, who first dila covered the erue manner in which the colours of the rainbow ale produced, of Galileo, of Kepler, and a few others, conftitute the subjects of this period; which is rendered peculiarly illustrious by ihe important discovery of those wonderful inftru: ments, the telescope and microscope; the effects of which, and the advantages which we derive from them, as the Author justly cblerves, are such as must appear a priori to be out of the reach of science to beítow, or of the human powers to produce.
With regard to the first of them, in particular, how improbable, even in idea, must it appear to any løber thinker, not familiarised, as we are, even from our infancy, with the powers of that instrument, that, by any contrivance whatever, a man tauld be enabled to annihilate, as it were, ninety nine hundredths or a much greater portion of the interval between him and an immensely distant, perhap: invisible, object; so as to view it nearly to the same advantage, as if he had actually in person traversed all this immenie space, and taken his station and reconnoitred it at the extremity of it:mand ghis, without stirring out of his parlour, and by such fender and unpromising means as a piece or two of glass or metal, formed and disposed în a particular manner! To one even previously acquainted with the magnifying power of lenses, when applied to obje&s near to them, the bare proposal to extend them to such purpoles must have appeared absurd, and the problem utterly impracticable.
The history of the discovery of this noble and useful inftru. ment (as well as of the microscope) is here minutely and agree. ably related. In this as well as in many other of the most useful and surprizing inventions, human genius has indeed little to boast, hou ever juftly it may pride itself in their subsequent extenfion and impro.ement, as the first discovery was intirely the work of chance. Independent of the historical testimony here collected, by which this fact is abundantly proved, it will evidently appear, from another consideration, that we owe the telescope to some 6
cafual combination of burning glasses, or spectacles ; as a confiderable time elapsed after the instrument had been constructed, and in the hands of many pertons, before the rationale of it, or the manner in which it produces its effects, was discovered and explained. We shail close what remains to be said on this subject, by an appofire and instructive reflection of the Author's, naturally függested by the circumstances of this extraordinary and unpremeditated invention ;. the history of which, he observes, furnishes a striking leflon to all philofophers, not to de spile the most trifling obfervation, or to withdraw their attention and study from thole powers of nature, or even those single facts, which may seem, at first fight, to be the most infignificant, and the most remote from every possible use. Every new fact, or property of any of the constituent parts of nature, ihould be carefully examined, as a treasure of unknown value, the real worth of wbich, time, and the discovery of other kindred powers in nature, may bring to light.'
In the third period are contained the observations and discoveries of Des Cartes, who very confiderably enriched the science of optics, of Snellius, who led the way to the discovery of the great law of refraction; of Scheiner, who first realised the ideas of Kepler, in constructing the astronomical and other te. lescopes, formed on plans different from that of the original or Galilean perspective ; and of Athanasius Kircher and others. In the Autbor's fourth period, which he considers as the true æra of the commencement of experimental philosophy, are comprehended all the observations made from the time of Des Cartes to that of Sir Isaac Newton. The principal heroes of this æra, among our countrymen, are Mr. Boyle, Dr. Gregory the original inventor of the reflecting telescope, Dr. Barrow, and the ingenious and indefatigable Dr. Hook: and, among Foreigners, Father Grimaldi, De la Hire, Mariotte, and Huygens.
All the discoveries hitherto made in the science of optics, however curious and interesting, were eclipled by the splendor and variety of those which form the subject of the next or fifth division of this work. After the exhibition of alt the more or less ingenious but random conjectures, with respect to the intimate nature of light, proposed or maintained by Plato, Ariftotle, Des Cartes, Grimaldi, Malebranche, and Dr. Hook, &c. but . unsupported by eyen a Shadow of evidence, or so much as an attempt at experimental proof,' the noble discovery of a true theory of light and colours by Sir Isaac Newton is here prelented, Itriking and beautiful in all its effential parts, and deduced from the clearest and most decisive experiments. It is curious to observe how much this great man was himself struck at his own success in this inquiry, and with what confidence this modeft and unafluming philosopher first announced this dis
covery in a letter preserved by Dr. Birch, and addressed to the Secretary of the Royal Society; where he terms it as being, in his judgment, “ the oddeft, if not the most confiderable de tection which hath hitherto been made in the operations of na
" • If this, says the Author, look like vanity in fo great a man, it is however the language of nature, and certainly very pardonable.' On this part of our Author's work we shall only observe, that he follows this great anatomift of light regularly through all the steps of his curious investigation, and gives a clear and connected view of all his profound discoveries on this subject : adding some ingenious and new observations or feveral of the more hypothetical parts of Sir Isaac's doctrine ; particularly on his attempt to account for the colours of thin plates, and all the other cases of the reflexion or transmiffion of light.
In the sixth and last period of this work, is contained, the hiltory of all the optical discoveries that have been made from the time of Sir Isaac Newton to the present. This short interval furnishes matter for the larger half of the work. It is in. deed a pretty generally received opinion, the Author obferves, shat scarce any advances have been made in the science of opcics fince the time of Sir Isaac Newton. But those who are perpetually expressing their wonder that no person has taken up the subject after him, and begun where he left off, the Author refers to the large and various contents of this division of his biftory; in which they will find that the philosophers of the present age have not been either backward or unsuccessful in prosecuting optical inquiries. Accordingly, far from having been obliged to make the most of his materials for this period, they have furnilhed him more exercise in the task of condenfing a great variety of matter into a small compass, than those of any other part of the work. The perural of this division of his hil tory will be particularly useful and agreeable to those who have been conversant only in the productions of Englith writers upon this subject, and will at the same time exemplify and confirm the utility of an undertaking of this kind; as the reader is thus brought acquainted with many curious and important observations which must be new to him, collected from the publications of the various philosophical societies establimed in different parts of Europe, or from the works of individuals, which have not been translated into our language. On account of the variety of new matter contained in this part of the work, we fhall dwell the longer upon it; first feleding, as a specimen, some of the observations of Mr. Bouger on the reflexion of light. Paffing over the ingenious and accurate methods here explained, by which he undertook to measure the degrees or intensity of emitted, refracted, or reflected light, we shall here briefly re
late fome of the deductions drawn from his curious and elabo. fare experiments in this new field of optical inquiry.
The most striking observations which he made, with respect to the reflexion of light from bodies, are those which relate to the very great difference produced in the quantity reflected, in consequence of a variation in the angle of incidence. This difference is truly surprizing between the great quantity of light reflected from the surface of water, at a small angle of incidence, and the very small portion reflected at a large angle At a very small angle of incidence water was found to reflect po lefs thao three-fourths of the incident light; that is, as much even as quicksilver,, which is of all known substances that which reflects the greateft quantity. On the other hand, the light reflected from water, at great angles of incidence, is exceeding small. Of a ray falling perpendicularly on its furface, it appears from all his observations that not more than the both, or rather the 55th part is reflected by it.
There is no person, this philofopher obferves, who has not felt the force of this strong reflexion, at small angles of incidence, from the smooth surface of a lake, when he has been walking, in still weather, on the brink opposite to the fun. . In this cafe the refe&ted light is , }, or sometimes a greater proportion of the light that comes directly from the fun, which is an addition to his direct rays that cannot fail to be very senfible. The direct light of the fun diminishes gradually, as it approaches the horizon, while the reflected light, at the fame time, grows stronger; so that there is a certain elevation of the sun, in which the united force of the direct and reflected light will be the greatest poslible, and this he says is 12 or 13 degrees.'
The difference in the quantity of light reflected from opaque bodies, after striking their surfaces with different degrees of obliquity, is almost as excessive as it is in the case of transparent substances. M. Bouger found this inequality to be the greatest in black marble ; • in which he was astonished to find that, with an angle of 3o. 35. of incidence, though not perfectly polished, yet it reflected almost as well as quicksilver. Of 1000 rays which it received, it returned 600; but when the angle of incidence was 15 degrees, it reflected only 156; when it was 30, it reflected 51; and when it was 80, it reflected only 23+
The angle of incidence is here every where supposed to be meafured from the reflecting furface itself, and not from the perpendiçular to it.
+ M. Boscovich accounts for the greater quantity of light reflected from bodies at great degrees of obliquity, bý observing tħar, in those The period of which we are now treating has not perhaps been distinguished by any discovery fo curious or important 2 that which gave rise to the late great improvement of the refracting telescope, which had been abandoned by Sir Ifaac Newton, on account of its supposed radical and incorrigible defects. As we have lately had frequent occations of treating this fubject *, we shall refer our Readers to the work itself for a full bistorical account of this matter. Our veneration, however, for the memory of our illustrious countryman will not permit us to pass over the ingenious attempt of the Author's friend, Mr. Michell (to whom he has been obliged for several other original and valuable communications inserted in this work) to ac,count for the remarkable overlight of that great man (in genesal fo peculiarly circumspect and accurate in conducting his experiments) in not having recognised the different differ fra power of various refracting substances; to the complete detec. tion of which quality in different media, by the late Mr. Dol. lond, we entirely owe a very capital discovery in the theory of light, and the consequent great improvements made in the refracting, now generally known by the name of the achromatic, or Dollond's telescope.
Mr. Michell observes, that as Sir Isaac used to put lucrbarum Jaturni into the water contained in his hollow prilinatie vetfel, (in that celebrated experiment of his wbich bas been the tub. ject of so much controversy) in order to increase its refractive power; the lead, even in this faline, form, might poffibly increase the dissipative refraction of the water, as it is now known to do in the glass, into the composition of which it enters. This, he observes, would account for Newton's not finding the disfipative power of water less than that of his glass prisms, which he otherwise ought to have done, if he had tried the experiment, as he said he did. Not content with merely supposing that the falt of lead inight produce this effect, Mr. Michell tried several experiments with a saturated foluti n of that fubftance in water; and the event appeared fully to juitify his conjecture on this head. We shall give the substance of the first of them.
He included a prism of glass, in a prism of water fully saturated with the salt of lead, and found that when the image, seen through both these prisms, was in its natural place, though it was still coloured, it was very slightly fo. He thought it was
circumstances, the perpendicular velocity of the ray is less ; fo that the force which refides at the reflecting surface can more easily, and with respect to more particles, destroy that velocity, and thereby determine the ray to be reflected.
In our accounts of the Memoirs of the Paris and Berlin Academies of Sciences, in our Appendixes for three or four years pait.