OPTIMISM 



ORACLE 



617 



besides pointing out the mode of constructing an 

 instrument better adapted for astronomical pur- 

 poses than that of Galileo ; he also made some 

 useful experiments on the nature of colours, and 

 showed that images formed on the retina of the 

 eye are inverted, a fact previously discovered by 

 Maurolycua of Messina. From this period the 

 science of optics steadily advanced, and its treasury 

 of facts received numerous additions through the 

 labours of De Doiuinis, Snell ( the discoverer of the 

 law of refraction in 1621 ), Descartes, Fermat, 

 Barrow, Mariotte, and Boyle. Up to the time of 

 Newton it was generally believed that colour was 

 produced \>\ refraction, but that philosopher showed 

 by a beautiful seiies of experiments that refraction 

 only separates the colours already existing in white 

 light. In his hands the theory and construction 

 ofthe telescope underwent many valuable improve- 

 ments, and in 1672 the description of his reflectiiuj 

 telescope was submitted to the Royal Society. 

 Gregory had constructed an instrument on similar 

 principles some years before. About the same 

 timeGrimaldi made his interesting series of experi- 

 ments on the effects of diffraction, and noticed the 

 remarkable fact of the interference of one pencil 

 of light with the action of another. The theory of 

 the rainbow, with an elegant analysis of the colours 

 of thin plates, and the hypothesis concerning the 

 nature and propagation of light, now known as 

 the 'corpuscular' theory (see LIGHT), completed 

 Newton's contributions to the science. The import- 

 ant services of the ingenious but eccentric Hooke 

 cannot lie easily stated in a brief alistract, as he 

 discovered a little of everything, completed nothing, 

 ami occupied himself to a large extent in combat- 

 ing faulty points in the theories of his contem- 

 poraries. It must not, however, be forgotten that 

 lie has as much right as Huygens to the credit of 

 originating the undulatory theory. The double 

 refraction of Iceland spar was discovered ( 1669) by 

 Bartholin, and fully explained in 1690 by Huygens, 

 the propounder of the undulatory theory, who also 

 aided the progress of mathematical optics to a 

 considerable extent. The velocity of light was 

 discovered by Homer (1675), and in 1720 the 

 al>erration of the fixed stars and its cause were 

 made known by Bradley, who likewise determined 

 with accuracy the amount of atmospheric refrac- 

 tion, liouguer, Porterfield, Euler, and Lambert 

 rendered essential service to physical optics ; the 

 same was done for the mathematical theory by 

 Dollond ( the inventor of the achromatic telescope), 

 Clairaut, D'Alembert, Boscovich, &C. ; while in later 

 times the experiments of Delaval on the colours 

 produced by reflection and refraction ; the discus- 

 sion of the phenomena arising from unusual reflec- 

 tion or refraction carried on Itv V'ince, Wollaston, 

 Biot, Monge, and others ; the discovery of polarisa- 

 tion of light by Mains (1808), and its investigation 

 by Brewster, Biot, and Seebeck ; of depolarisation 

 by Arago (1811), and of the optical properties as 

 connected with the axes of crystals (1818) by 

 Brewster ; and the explanation of these and other 

 optical phenomena in accordance with the undula- 

 tory li.y|Kithet)iii by Young the discoverer of the 

 Interference (q.v.) of rays and Fresnel, went far to 

 give optics a width of scope and a symmetry which 

 are possessed by few other sciences. The develop- 

 ment of the undulatory theory and of optical 

 science generally has l>een carried on in the present 

 century by Lloyd, Airy, Cauchy, C'lerk-Maxwell, 

 Hertz, ami others ; and for an account of the 

 present state of the science reference may be made 

 to Mr Thomas Preston's Theory of Light" ( 1890), in 

 addition to the works mentioned under LIGHT, and' 

 the articles LENSES, MICROSCOPE, TELESCOPE, &c. 

 Optimism ( Lat. optimut, 'best'), the doctrine 

 that the existing order of things, whatever may be 



its seeming imperfections of detail, is nevertheless, 

 as a whole, the most perfect or the best which 

 could have been created, or which it is possible to 

 conceive. Some of the advocates of optimism 

 content themselves with maintaining the absolute 

 position, that, although God was not by any means 

 bound to create the most perfect order of things, 

 yet the existing order is de facto the best ; others 

 contend that the perfection and wisdom of Almighty 

 God necessarily require that His creation should bo 

 the most perfect which it is possible to conceive. 

 The philosophical discussions of which this con- 

 troversy is the development are as old as philo- 

 sophy itself, and are dealt with in the article on 

 the origin of Evil (q.v.). But the full development 

 of the optimistic theory as a philosophical system 

 was reserved for Leibnitz (q.v.), in his Theutlii -r, , 

 the main thesis of which is that, among all the 

 systems which presented themselves to the infinite 

 intelligence of God as possible, God selected and 

 created in the existing universe the liest and most 

 perfect, physically as well as morally, regard being 

 Iiad to the universe as a whole. The Theodicee was 

 designed to meet the sceptical theories of Bayle, 

 and its theories were ridiculed in Voltaire's Cuntlitle. 

 Modern discussion on this question usually assumes 

 the form of assertion or denial of the opposite 

 doctrine of Pessimism (q.v.). 



Opuntia. Sec PRICKLY PEAR. 



Opus OperatlHIl (Lat., literally 'the work 

 wrought ') is the phrase employed in the Catholic 

 theological schools to describe the manner of opera- 

 tion of the sacramental rites in the production of 

 grace. It is intended to imply that the ministra- 

 tion of the rite (opus) is in" itself, through the 

 institution of Christ, an efficient cause of grace, 

 and that, although its operation is not infallible, 

 but requires and presupposes certain dispositions 

 on the part of the recipient, yet these dispositions 

 are but tonditionet sine qua non, and do not of 

 themselves produce the grace. Hence, when the 

 sacraments are administered to dving persons in a 

 state of apparent insensibility, tiiis is done in the 

 hope and on the presumption that the dying pei'son 

 may, though seemingly unconscious, be neverthe- 

 less really disposed to receive the sacrament ; but 

 it is by no means held that if these dispositions be 

 wanting the sacrament will itself justify him. The 

 phrase Opus Operantis is frequently used as denoting 

 that the effect of a particular ministration or rite 

 is primarily and directly due, not to the rite itself 

 (opus), but to the dispositions of the recipient 

 (uperans). Thus, in the act of kissing or praying 

 before a crucifix, of sprinkling one's self with holy 

 water, of telling the prayers of the rosary upon 

 blessed beads, the fervour and personal piety of 

 the supplicant, and not the material object of the 

 religious use, is held to be the efficient cause of the 

 grace which is thereby imparted. 



Orjichc (Atriplex), a genus of plants of the 

 natural order Chenopodiacete, having male and 

 female flowers on the same plant. The species are 

 numerous and widely spread. over the maritime or 

 saline parts of the earth, scarcely any species 

 except the Common Orache (A.patula) being ever 

 found inland or away from saline influence. Five 

 species, including the Garden Orache (A. hortensis), 

 are natives of Britain. Although formerly much 

 cultivated in Britain, orache is now displaced as 

 a pot-herb by spinach (Spinacia oleracea),& species 

 of a closely allied genus. All the species have 

 .similar qualities, and may be used as spinach. 



Orilde, the response delivered by a deity or 

 supernatural being to a worshipper or inquirer ; 

 also the place where the response was delivered. 

 These responses were supposed to be given by a 

 certain divine afflatus, either through means of 



