Rainbow, knowledge of the nature of, a modern discovery, 211. Approach towards it by Fletcher of Brellaw, 212. The discovery of, made by Antonio de Dominis bishop of Spalatro, 213. True cause of its colours, 214. Phenomena of the rainbow explained on the principles of Sir Isaac Newton, 215. Two rainbows seen at once, 216. Why the arc of the primary rainbow is never greater than a semicircle, 217. The secondary rainbow produced by two reflections and two refractions, 218. Why the colours of the secondary rainbow are fainter than those of the primary, and ranged in a contrary order, 219.
Ramden's, Mr., new equatorial telescope, 89.
Rays of light extinguished at the surface of transparent bodies, 37. Why they seem to proceed from any luminous object when viewed with the eyes half shut, 50. Rays at a certain obliquity are wholly reflected by transparent substances, 115. The focus of rays refracted by
N
Newton, Sir Isaac, his discovery concerning colours, 16. Mistaken in one of his experiments, 18. His discoveries concerning the inflection of light, 51. Theory of refraction objected to, 121. These objections are the necessary consequences of the theory, and therefore confirm it, 122. Reflecting telescope, 257. Magnifying power of, 259. Inferior to Gregorian, 265.
Nollet, Abbé, cannot fire inflammable liquids by burning glasses, 43.
Objects on the retina of the eye appear inverted, 133. Why seen upright, 134. An object when viewed with both eyes does not appear double, because the optic nerve is insensible of light, 135. Proved by experiments, 136. Seen with both eyes brighter than when seen only with one, 150. The various appearances of objects seen through different media stated and investigated, 152. An object situated in the horizon appears above its true plane, 153. An object seen through a plane medium appears nearer and brighter than seen by the naked eye, 154. Object seen through a convex lens appears larger, brighter, and more distant, 157. In some circumstances an object through a convex lens appears inverted and pendulous in the air, 156. Barrow's theory reflecting the apparent place of objects, 181. M. de la Hire's observations, 182. M. le Cat's account of the largeness of objects in mist, 183. Why objects seen from a high building appear smaller than they are, 189. Dr Porterfield's account of objects appearing to move to a giddy person when they are both at rest, 193. Wells's account, 194. Upon what data we judge visible objects to be in motion or at rest, 195. Experiments to ascertain it, 196.
Object-glasses improved by Dollond, 17, and by Blair, 19.
Observatory, portable. See Equatorial Telescope.
Opaque objects, microscope for, 103.
Optic nerve insensible of light; and therefore an object viewed by both eyes is not seen double, 135. Proved by experiments, 136.
Optical instruments, p. 267.
M
Mauclerc, his discoveries, 9, 63.
Mazeas, Abbé, attempts to explain the phenomena of green and blue shadows, 226.
Media, the various appearance of objects through different, stated and investigated, 152. An object seen through a plane medium, appears nearer and brighter than seen by the naked eye, 154.
Melville, Mr., his observations on the heating of bodies by light, 42. Discovers that bodies which seem to touch are not in actual contact, 45. Explains a curious phenomenon of vision, 198. Explains the phenomena of green and blue shadows in the sky, 227.
Mitchell's, Mr., calculation of the light of the moon, 272.
Microscopes, their history, 92. Made by Janßen, 93. By Divini, 94. By Hartsoeker, 95. By Leeuwenhoek, 96. By Wilton, 97. Adam's method of making globules for large magnifiers, 98. Temporary microscopes, by Mr Grey, 99. Varnish ones, by Dr Brewster, 100. Dr Barker's reflecting microscope, 101. Smith's reflecting microscope superior to all others, 102. Solar microscopes and that for opaque objects, 103. Mr Euler's scheme of introducing vision by reflected light into the solar microscope and magic lantern, 104. Martin's improvement, 105. Di Torre's extraordinary magnifying microscope, 106. Could not be used by Mr Baker, 107. Microscope compound, use of several lenses in, 242. Dr Smith's magnifying power of, 244. Easy method of ascertaining the magnifying power of, 245. Further observations on the magnifying power of, 246. Table of the magnifying powers of glasses used in, ib. Solar, magnifying power of, 248. Merits of, compared with the telescope, 263.
Mines better illuminated in cloudy than in clear weather, 46.
Mirrors, size of, in which a man may see his whole image, 239. Why three or four images of objects are seen in plane mirrors, 140. Aerial images formed by concave mirrors, 241.
Moly, account of the largeness of objects in, by M. le Cat, 183.
Moon, why visible when totally eclipsed, 236. Why the moon appears duller when eclipsed in her perigee than in her apogee, 237. Great variation of the light of the moon at different altitudes, No. 268. M. Bouguer's calculations concerning the light of, 270. Dr Smith's, 271. Mr Mitchell's, 272.
Motion of light accelerated or retarded by refraction, 114.
Multiplying glasses, 238.
O
Objects on the retina of the eye appear inverted, 133. Why seen upright, 134. An object when viewed with both eyes does not appear double, because the optic nerve is insensible of light, 135. Proved by experiments, 136. Seen with both eyes brighter than when seen only with one, 150. The various appearances of objects seen through different media stated and investigated, 152. An object situated in the horizon appears above its true plane, 153. An object seen through a plane medium appears nearer and brighter than seen by the naked eye, 154. Object seen through a convex lens appears larger, brighter, and more distant, 157. In some circumstances an object through a convex lens appears inverted and pendulous in the air, 156. Barrow's theory reflecting the apparent place of objects, 181. M. de la Hire's observations, 182. M. le Cat's account of the largeness of objects in mist, 183. Why objects seen from a high building appear smaller than they are, 189. Dr Porterfield's account of objects appearing to move to a giddy person when they are both at rest, 193. Wells's account, 194. Upon what data we judge visible objects to be in motion or at rest, 195. Experiments to ascertain it, 196.
Object-glasses improved by Dollond, 17, and by Blair, 19.
Observatory, portable. See Equatorial Telescope.
Opaque objects, microscope for, 103.
Optic nerve insensible of light; and therefore an object viewed by both eyes is not seen double, 135. Proved by experiments, 136.
Optical instruments, p. 267.
P
Parallel rays falling perpendicular upon any lens, the focus of, found, 130.
Parhelion, p. 262.
Photometer, Rumford's, No. 273. Saufure's, p. 287. Leflic's, p. 288.
Plane medium, an object seen through appears nearer and brighter than by the naked eye, No. 154.
Plane surfaces, laws of refraction in, 127. Extent of the visible horizon on, 220.
Planets more luminous at their edges than in the middle of their disks, 49, 269.
Plates. Maraldi's experiments concerning their shadows, 55.
Porta, Joannes Baptista, his discoveries, 10.
Porterfield's solution of single vision with two eyes, 147. Of the judging of the distance of objects, 188. Fallacies of vision explained, 190. Porterfield's account of objects appearing to move to a giddy person when they are both at rest, 193.
Primary rainbow never greater than a semicircle, and why, 217. Its colours stronger than those of the secondary, and ranged in contrary order, 219.
Prisms in some cases reflect as strongly as quicksilver, 38. Why the image of the sun by heterogeneous rays passing through a prism is oblong, 227.
Ptolemy first treated of refraction scientifically, 4.
Rainbow, knowledge of the nature of, a modern discovery, 211. Approach towards it by Fletcher of Brellaw, 212. The discovery of, made by Antonio de Dominis bishop of Spalatro, 213. True cause of its colours, 214. Phenomena of the rainbow explained on the principles of Sir Isaac Newton, 215. Two rainbows seen at once, 216. Why the arc of the primary rainbow is never greater than a semicircle, 217. The secondary rainbow produced by two reflections and two refractions, 218. Why the colours of the secondary rainbow are fainter than those of the primary, and ranged in a contrary order, 219.
Ramden's, Mr., new equatorial telescope, 89.
Rays of light extinguished at the surface of transparent bodies, 37. Why they seem to proceed from any luminous object when viewed with the eyes half shut, 50. Rays at a certain obliquity are wholly reflected by transparent substances, 115. The focus of rays refracted by by spherical surfaces ascertained, No 128. The focus of parallel rays falling perpendicularly upon any lens, 130. Emergent rays, the focus of, found, 131. Rays proceeding from one point and falling on a parabolic concave surface are all reflected from one point, 174. Proportional distance of the focus of rays reflected from a spherical surface, 175. Several sorts of coloured rays differently refrangible, 204. Why the image of the sun by heterogeneous rays passing through a prism is oblong, 207. Every homogeneous ray is refracted according to one and the same rule, 210.
Reflected light, table of its quantity from different substances, 39.
Reflecting telescope of Newton, 257. Magnifying power of, 259.
Reflection of light, opinions of the ancients concerning it, 23. Bouguer's experiments concerning the quantity of light lost by it, 32. Method of ascertaining the quantity lost in all the varieties of reflection, ib. Buffon's experiments on the same subject, 33. Bouguer's discoveries concerning the reflection of glass and of polished metal, 34. Great difference of the quantity of light reflected at different angles of incidence, 35. No reflection but at the surface of a medium, 42. Rays at a certain obliquity are wholly reflected by transparent substances, 115. Total reflection produced by the brilliant cut in diamonds, 116. Some portion of light always reflected from transparent bodies, 118. Light is not reflected by impinging on the solid parts of bodies at the first surface, 159; nor at the second, 160. Fundamental law of reflection, 160. Laws of, from a concave surface, 170. From a convex, 171. These preceding propositions proved mathematically, 172. Reflected rays from a spherical surface never proceed from the same point, 173. Rays proceeding from one point and falling on a parabolic concave surface are all reflected from one point, 174. Proportional distance of the focus of rays reflected from a spherical surface, 175. Method of finding the focal distance of rays reflected from a convex surface, 176. The appearance of objects reflected from plane surfaces, 177; from convex, 178; from concave, 179. The apparent magnitude of an object seen by reflection from concave surface, 180. Reflected light differently refrangible, 205.
Refracting telescopes improved by Mr Dollond, 17. By Dr Blair, 19. Magnify in proportion to their lengths, 255. Imperfections in, remedied, 256.
Refraction, known to the ancients, 2. Its laws discovered by Snellius, No 11. Explained by Descartes, 12—Fallacy of his hypothesis, 13. Experiments of the Royal Society for determining the refractive powers of different substances, ib.—M. de la Hire's experiments on the same subject, ib. Refraction of air accurately determined, 13, 14. Mistake of the Academy of Sciences concerning the refraction of air, 13. Allowance for refraction in computing the height of mountains, first thought of by Dr Hooke, 14. Mr Dollond discovers how to correct the errors of telephones arising from refraction, 17. The same discovery made by Mr Hall, 18. Important discovery of Dr Blair for this purpose, 19. Refraction defined, 111. Phenomena of refraction solved by an attractive power in the medium, 112. Refraction explained and illustrated, pages 206, 207, &c.—Ratio of the fine of incidence to the fine of refraction, No 113. Refraction accelerates or retards the motion of light, 114. Refraction diminishes as the incident velocity increases, 117. Refraction of a star greater in the evening than in the morning, 118. Laws of refraction when light passes out of one transparent body into another contiguous to it, 120. The Newtonian theory of refraction objected to, 121. Which objections, as they are the necessary consequences of that theory, confirm it, 122. Laws of refraction in plane surfaces, 127. The focus of rays refracted by spherical surfaces ascertained, 128. Light consists of several sorts of coloured rays differently refrangible, 194.—Reflected light differently refrangible, 205. Every homogeneous ray is refracted according to one and the same rule, 210.
Reid's solution of single vision with two eyes, 148.
Repulsive force supposed to be the cause of reflection, 161. Objected to, 162. Another hypothesis, 166. Sir Isaac Newton's, 167. Untenable, 168.
Retina of the eye, objects on, inverted, 133. Why seen upright, 134. When viewed with both eyes, not seen double, because the optic nerve is insensible of light, 135. Arguments for the retina's being the seat of vision, 139.
Rheita's telescope improved by Huygens, 79. His binocular telescope, 80.
Robins', Mr, objection to Smith's account of the apparent place of objects, 186.
Saturn's ring discovered by Galileo, 72. Secondary rainbow produced by two reflections and two refractions, 218. Its colours why fainter than those of the primary, and ranged in contrary order, No 219.
Schleiner completes the discoveries concerning vision, 64. Puts the improvements of the telescope by Kepler in practice, 78.
Shadows of bodies, observations concerning them, 47, 48, 49. Green shadows observed by Buffon, 224. Blue ones, 225. Explained by Abbé Mazzea, 226.—Explained by Melville and Bouguer, 227. Curious observations relative to this subject, 228. Blue shadows not confined to the mornings and evenings, 229.—Another kind of shadows, 230. Illumination of the shadow of the earth by the refraction of the atmosphere, 236.
Short's, Mr, equatorial telescope, 89.
Short-livedness, 142.
Sky, concave figure of, p. 262, &c. Why the concavity of the sky appears less than a semicircle, No 222. Opinions of the ancients reflecting the colour of the sky, 223. New explanation of its blue colour, 231.
Smith's, Dr, reflecting microscope superior to all others, 102. Account of the apparent place of objects, 185. Objected to, 186. Converging irradiation of the sun observed and explained by, 232, 233. He never observed them by moonlight, 234. Diverging beams more frequent in summer than in winter, 235. Calculation concerning the light of the moon, 271. His microscope, magnifying power of, 244.
Solar microscope, 103. Mr Euler's attempt to introduce vision by reflected light into the solar microscope, 104. Martin's improvement, 105. Magnifying power of, 248.
Spectacles, when first invented, 67.
Specula for reflecting telephones, how to grind and polish them, 285.
Spots of the sun discovered by Galileo, 72. Not seen under so small an angle as lines, 144.
Stars, how to be observed in the daytime, 90. The refraction of a star greater in the evening than in the morning, 118.
Sun, image of, by heterogeneous rays passing through a prism, why oblong, 207. The image of, by simple and homogeneous light, circular, 208. Variation of light in different parts of the sun's disk, 269.
Surfaces of transparent bodies have the property of extinguishing light, and why, 37. Supposed to consist of small transparent planes, 39, 41. Laws of refraction in plane surfaces, 127. The focus of rays refracted by spherical surfaces ascertained, 128. Reflected rays from Index.
**OPTICS**
a spherical surface never proceed from the same point, No 173. The appearance of objects from plane surfaces, 177. From convex, 178. From concave, 179. The apparent magnitude of an object seen by reflection from a concave surface, 180.
**T**
Telescopes: different compositions of glass for correcting the faults of the refracting ones, 18. Descartes's account of the invention of them, 63. Other accounts, 69. The first one exceeding good, 70. Galileo made one without a pattern, 71. His discoveries on this head, 72. Account of his telephones, 73. Rationale of the telescope first discovered by Kepler, 74. Reason of the effects of telescopes, 75. Galilean telescope difficult of construction, 76. Telescopes improved by Kepler, 77. His method first practised by Scheiner, 78.—Huygens improves the telephones of Scheiner and Rheita, 79. Rheita's binocular telescope, 80. Telescopes of Campani and Divini, 81. Azout makes a telescope of an extraordinary focal length, 82. Telescopes used without tubes, 83. On the apertures of refracting telescopes, 84. History of the reflecting telescope, 85. Mr Edwards's improvements in it, 86. Herschel's improvements, 87. Mr Dollond's improvements, 88. The equatorial telescope, 89. How to observe stars in the daytime, 90. Mr Epinus's proposal for bending the tubes of telescopes, 91. Telescope, astronomical, 249. Magnifying power of, 250. Inverts objects, 251. Common refracting, shows objects erect, 252. Galilean telescope, No 253. Magnifying power of, 254. Refracting, magnify in proportion to their length, 255. Achromatic ones, 256. Reflecting telescope of Newton, 257. Magnifying power of, 259. Gregorian telescope, 260. Magnifying power of, 261. Cassegrainian telescope, 262. Merits of, compared with the microscope, 263. Refracting telescopes improved by Dollond and Blair, superior to all others, 264. Gregorian telescope superior for common uses to the Newtonian, 265.
Thin plates; Mr Boyle's account of the colours observable in them, 30. Dr Hooke's account, 31. Thomson's, Mr, portable camera obscura. Torre's, F. di, extraordinary magnifying microscope, 156.
Tour, M. de, his observations on the inflection of light, 58. The hypothesis by which he accounted for the phenomena, 59. Unsatisfactory and ill-founded, 60.
Transparent bodies, a portion of light always reflected from, 158.
**V**
Variation of the intensity of attraction and repulsion unknown, 125. The law of variation in the actions of many particles different from that of one; but may be known if it be known, 126. Variation of the light of the moon at different altitudes, 268. In different parts of the disks of the sun and planets, 269.
Visible objects how judged to be in motion or at rest, 195. Curious experiments to ascertain it, No 196. Visible horizon on a plane surface, extent of, 220.
Vision: its nature first discovered by Maurolycus, 9. Discoveries concerning it, p. 193. Seat of, dispute about, No 137. Dimensions of the spot in the eye where there is no vision, 138. Arguments for the retina's being the seat of vision, 139. Vision bright and obscure, 140. Distinct at different distances, 141. Least angle of vision, 156. Of single vision with two eyes, 145. Briggs's solution, 146. Porterfield's, 147. Reid's, 148. Wells's, 149. Vision more distinct in homogeneous than heterogeneous light, 209. Several fallacies of vision explained, 190. Great light thrown on this subject by M. Bouguer, 191. A remarkable deception explained by M. le Cat, 197. Curious phenomenon explained by Mr Melville, 198.
Vitellio's discoveries, 7.
**U**
Undulation, Euler's theory of, contrary to fact, 123; and millshead artifices, 124.
**W**
Water in some cases reflects more powerfully than quicksilver, 35. Table of the quantity of light reflected from it at different angles, ib. Remarkably strong reflection into it from air, 36.
Well's solution of single vision with two eyes, 149. Accounts for objects appearing to move to a giddy person when at rest, 194.
White bodies reflect more light than others, 28.
Wilson's microscope, 97.
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**OPTIMATES**
Optimates, one of the divisions of the Roman people, opposed to populares. It is not easy to ascertain the characteristic differences betwixt these two parties. Some say the optimates were warm supporters of the dignity of the chief magistrate, and promoters of the grandeur of the state, who cared not if the inferior members suffered, provided the commanding powers were advanced: Whereas the populares boldly stood up for the rights of the people, pleaded for larger privileges, and laboured to bring matters nearer to a level. In short, they resembled, according to this account, the court and country parties amongst the people of this island.
Tully says, that the optimates were the best citizens, who wished to deserve the approbation of the better sort; and that the populares courted the favour of the populace, not so much considering what was right, as what would please the people and gratify their own thirst of vain glory and empty applause.
**OPTIO**, an officer in the Roman army, being an assistant or lieutenant to every centurion. The optio was so called because he was the choice or option of the centurion in later times; at first, however, he had been chosen by the tribune, or chief commander of the legion. These optiones are also sometimes called succenturiates and tergitudinarii; the last name was given them because their post was in the rear of the company. Some authors make mention of sub-optiones or sub-lieutenants.
It is proper, however, to add, that optiones were not peculiar to the camp, but were also used in a variety of other offices of life.
**OPTION**, the power or faculty of wishing, or choosing; or the choice a person makes of any thing.
When a new suffragan bishop is consecrated, the archbishop of the province, by a customary prerogative, claims the collation of the first vacant benefice, or dignity, in that see, according as he shall choose; which choice is called the archbishop's option.
But in case the bishop dies, or is translated, before the the present incumbent of the promotion chosen by the archbishop shall die or be removed, it is generally supposed that the option is void; inasmuch as the grantor, singly and by himself, could not convey any right or title beyond the term of his continuance in that see. And if the archbishop dies before the avoidance shall happen, the right of filling up the vacancy shall go to his executors or administrators.