616 



OPOTECA 



OPTICS 



there is no |xmch. The Virginian Opossum U a 

 foe to poultry-yards in tlir I'nited state- ; hut it 

 can put up with fmgs if there is nothing better to 

 be hail. The opossums, like other uiarsii|)ial8, liave 

 a lowly organised I. ruin ; hut tin-vans remarkably 

 cunning in robbing poultiv yards ; on the other 

 liiLinl. thfir stupidity in walking straight into the 

 simplest and most ohvious trap i- mom in accord 

 with their brain structure. Hunting the opossum 

 with dogs by night is a favourite sport in the 

 southern states, especially in autumn, when the 

 body has a thick layer of fat all over. The animal 

 takes refuge in a tree, and is either shaken down 

 or shot as it hangs by the tail. The expression 

 ' playing possum ' refers to the opossum s habit 

 ot feigning death when caught. At such times, 

 though usually very timid, it will endure almost 

 any amount of torture, and give no sign of its 

 suffering. 



Opoteca a sleepy town of 1000 inhabitants, in 

 Honduras, alxiut 15 miles NjiW. of Comayagua, 

 formerly famous for its great silver- mines. 



OppHll. a town of Prussian Silesia, on the 

 Oder, 51 miles SE. of Rreslau. Since 1816, when 

 it was erected into a seat of government for Upper 

 Sili ia. the town has been much beautified ooth 

 with new edifices and with parks and gardens. 

 IU church of St Adalbert was founded in 995 ; 

 and there is an old castle on an island in the Oder. 

 The manufactures include pottery, cigars, cement, 

 beer, leather, &c., and there is a considerable trade 

 in grain and cattle. Pop. (1875) 12,498; (1885) 

 15,975; (1890)19,206. See Idzikowski's Ucsckichte 

 derStadt Oppeln ( 1863). 



OppriiliHm. a town of Hesse-Darmstadt, on 

 the left hank of the Rhine, 20 miles SSE. of Mainz 

 by rail, with fine vineyards. On the site of the 

 Human castle of Baucunica, Oppenhcim became a 

 free city of the empire, and was repeatedy besieged, 

 especially in the Thirty Years' War. Pop. 3452. 



Opportunists, in French politic?, are those 

 who, like Gamlietta, Ferry, and others like-minded, 

 oppose doctrinaire as well as extreme views, accom- 

 modate themselves in great measure to the cir- 

 cumstances of the hour, and aim only at what can 

 obviously be carried through. 



Optical Illusion. An object appears large 

 or small, near or distant, according as the rays 

 from ite opposite bottlers meeting at the eye form 

 a large or a small angle : when the angle is large, 

 the ol pji-rt is either large or near ; when small, the 

 object mn-t ! small or distant. Experience alone 

 enables us to decide whether an object of large 

 apparent size is so on account of its real size, or of 

 it- proximity ; and our decision is arrived at by a 

 comparison of the object in position with other 

 common objects, such as trees, houses, &c., which 

 may chance to be near it, and of which we have by 

 experience come to form a correct idea. The same 

 is, of course, true of apparently small objects. But 

 when all means for comparison are removed our 

 judgment is at fault. Similarly, we erroneously 

 infi-r spherical solids at a distance to be flat discs ; 

 and, by reason of Irradiation (n.v.) in the eye, the 

 sun appears larger than In- would if illumined by a 

 fainter light, and a man in a white habit serins 

 larger than he would if he wore a dark dress. 

 Hill-inns are also produced by external can- - : 

 and instanced of this sort arc given under MIKAOK, 

 1 1' >.N, and KKFK ACTION. 



The persistence of impn-ssions on the retina for 

 about one-sixth of a second after the object which 

 produced the impression has been removed produces 

 another class of illusions. Common examples of 

 this are tin- illuminated ciieh- tniim-d by tlic iapid 

 evolution of an ignited earlxin point, piece of red 

 hut iron, or other luminous body, and the fiery 



curve produced by a red-hot shot projected from a 



cannon. 



Another form of illusion is produced to a person 

 who is seated ill a vehicle in motion ; and it is very 

 deceptive when th<> motion is so equable as not to 

 be felt by the person himself. The illusion is most 

 complete when the attention is riveted on an object 

 several yards oil': this object then appears to be 

 a centre round which all the other objects re- 

 volve, those ln'twccn the observer and the ol. 

 moving backwards, and those beyond the object 

 moving forwards. This illusion occurs on a large 

 scale in the apparent motion of the heavenly 

 bodies. Other illusions arise from H disoidcr.-d 

 state of the organs of vision : e.g. the seeing of 

 things double or movable, or of a colour dillerent 

 from the tnie one (see C'oi.ot K-III.INDNESS) ; the 

 appearance as of insects crawling over a body at 

 which the eye is directed, &c. 



Optic Xcrve. See EVE. 



Optics is the science of the phenomena of 

 light. This science i- usually treated under to 

 heads : ( 1 ) Physical Optics, which treats of the 

 nature of Light (q.v., as also MAGNETISM and 

 I \nri.ATouY THEORY), and explains the pheno- 

 mena of Colour, Reflection, Refraction, Interfer- 

 ence and its consequences, such as the colours of 

 thin plates and films, Diffraction, Dispersion, the 

 Spectrum, Polarisation and the properties of ]Kilar- 

 i-fd light, for which see separate articles ; and 

 ('2) Geometrical or Mathematical Optics. The 

 leading idea in physical optics is to trace the 

 progress of an undnlatory or oscillatory disturbance 

 in the Ether (q.v.) ; this disturbance, which may 1 

 termed a wave, has an advancing wave-front ; the 

 direction along which this wave- front advai. 

 through a given point is a geometrical conception, 

 which it is convenient to make use of in diagrams, 

 more convenient than it would l>e to draw a seiies 

 of successive wave-fronts ; this direction of propa- 

 gation through any given point is called a ray ; and 

 geometrical optics traces, by mathematical reason- 

 ing, the course of a given set of 'rays' under 

 specified conditions, particularly under those which 

 have reference to Reflection and Refraction (q.v.). 

 The part of geometrical optics which deals with 

 reflection of light is often called Catoptrics (based 

 on such laws as that the angle of reflection is equal 

 to the angle of incidence) ; that which deals ith 

 refraction is called Dioptrics : and for an account 

 of these, reference is made to articles REFLECTION 

 and REFRACTION respectively. 



Though the Creeks and their disciples the Arabs 

 had made some progress in mathematical optics, 

 their knowledge was confined to the law of lellection 

 and its more immediate consequences. Euclid, 

 Aristotle, Archimedes, Hero, and Ptolemy were 

 acquainted with the fact that light is transmitted 

 in straight lines; but, with the important exception 

 of Aristotle and some of his followers, the ancient 

 philosophers believed that ra\s proceeded /mm the 

 e\e In the object, instead of'in the contrary direc- 

 tion. Ptolemy was well acquainted with atmo- 

 spheric refraction. Alhacen ( 1070) and Vitellio the 

 Pole (1260) were almost the only cultivators of this 

 science during the middle ages, and their additions 

 to it were unini]>ortant. The lens, though known 

 from early antiquity, was not applied as an aid to 

 defective 'eyesight till after the time of Roger 

 Bacon. Jansen. Metin-, and Galileo separately 

 invented the telescoix- aliout the beginning of the 

 17th century; and tin- la.-I mentioned philosophci 

 by its means made various important astronomical 

 discoveries. Kepler, a short time after, gave the 

 tin.- theory of the telescope, explained the method 

 of finding 'the focal length of lenses, and iipplh d it 

 to find the magnifying power of the telescope 



