OPT 



01 J T 



OPOPANAX. See GUM resln> 



OPOSSUM. See DIDELPHIS. 



OPTICS, the science of vision, includ- 

 ing Catoptrics and Dioptrics, and even 

 Perspective ; as also the whole doctrine 

 of light and colours, and all the pheno- 

 mena of visible objects. See PERSPEC- 

 TIVE. 



Optics, in its more extensive accepta- 

 tion, is a mixed mathematical science, 

 wlu'ch explains the manner in which vi- 

 sion is performed in the eye ; treats of 

 sight in general ; gives the reasons of the 

 several modifications or alterations which 

 the rays of light undergo in the eye ; and 

 shows why objects appear sometimes 

 greater, sometimes smaller, sometimes 

 more distinct, sometimes more confused, 

 sometimes nearer, and sometimes more 

 remote. In this extensive signification it 

 is considered by Sir Isaac Newton, in his 

 Optics. Indeed optics make a consider- 

 able branch of natural philosophy ; both 

 as it explains the laws of nature, accord- 

 ing to which vision is performed, and as 

 it accounts for abundance of physical phe- 

 nomena, otherwise inexplicable. 



The reflection of the rays of light is, 

 indeed, an occurrence too frequent and 

 obvious to have escaped the notice even 

 of the earliest observers ; a river or some 

 other piece of water was probably the 

 first mirror ; its effect was afterward 

 imitated by metallic mirrors : hence was 

 discovered the equality of the angles of 

 incidence and reflection. It was known 

 at an early period that an oar, or other 

 straight piece of wood, partially immers- 

 ed in water, no longer appeared straight, 

 yet ages after this elapsed before any at- 

 tempts were made to ascertain the rela- 

 tion between the angles of incidence and 

 refraction. Empedocles was the first 

 person on record that wrote systemati- 

 cally on light; and Euclid composed a 

 treatise on the ancient optics and catop- 

 trics ; dioptrics being less known to the 

 ancients, though it was not quite unno- 

 ,ticed by them, for among the phenomena 

 at the beginning of that work, Euclid re- 

 marks the effect of bringing an object in- 

 to view, by refraction, in the bottom of a 

 vessel, by pouring water into it, which 

 could not be seen over the edge of the 

 vessel before the water was poured in ; 

 and other authors speak of the then 

 known effects of glass globes, &c. both 

 as burning glasses, and as to bodies seen 

 through them. Euclid's work, the genu- 

 ineness of which has been doubted, is 

 chiefly on catoptrics, or reflected rays ; 

 in which he shows the chief properties of 

 them in plane, convex, and concave sur- 



faces, in his usual geometrical manner, 

 beginning with that concerning the equa- 

 lity of the angles of incidence and re- 

 flection, which he demonstrates ; and in 

 the last proposition, showing the effect 

 of a concave speculum, as a burning 

 glass, when exposed to the rays of the 

 sun. 



The effects of burning glasses, both by 

 refraction and reflection, are noticed by 

 several others of the ancients, and it has 

 been thought that the Romans had a me- 

 thod of lighting their sacred fire by some 

 such means. Aristophanes, in one of his 

 comedies, introduces a person as making 

 use of a globe filled with water to cancel 

 a bond that was against him, by thus melt- 

 ing the wax of the seal. If we give cre- 

 dit to what some ancient historians are 

 said to have written concerning the ex- 

 ploits of Archimedes, we shall be induced 

 to think that he constructed some very 

 powerful burning mirrors. It is even al- 

 lowed that this eminent geometrician 

 wrote a treatise on the subject of them, 

 though it be not now extant ; as also con- 

 cerning the appearance of a ring or cir- 

 cle under water, and therefore could not 

 have been ignorant of the common phe- 

 nomena of refraction. We find many 

 questions concerning optical appearances 

 in the works of Aristotle. This author 

 was also sensible that it is the reflection 

 of light from the atmosphere which pre- 

 vents total darkness after the sun sets, 

 and in places where he does not shine in 

 the day time. He was also of opinion, 

 that rainbows, halos, and mock suns were 

 all occasioned by the reflection of the 

 sun-beams in different circumstances, by 

 which an imperfect image of his body 

 was produced, the colour only being ex- 

 hibited, and not his proper figure. The 

 ancients were not only acquainted with 

 the more ordinary appearances of refrac- 

 tion, but knew also the production of co- 

 lours by refracted light. Seneca says, 

 that when the light of the sun shines 

 through an angular piece of glass, it 

 shows all the colours of the rainbow. 

 These colours, however, he says, are 

 false, such as are seen in a pigeon's neck 

 when it changes its position ; and of the 

 same nature, he says, is a speculum, 

 which, without having any colour of its 

 own, assumes that of any other body. 



It appears also, that the ancients were 

 not unacquainted with the magnifying 

 power of glass globes filled with water, 

 though they probably knew nothing of 

 the reason of this power ; and it is sup- 

 posed that the ancient engravers made 

 use of a glass globe filled with water to 



