92 LIGHT. 



LIGHT. 



Op'tics ((jrr. opfikos, pertaining to seeing). — The science which 

 treats of the laws of light and vision. 



Aberra'tion. — A deviation in the rays of light, when inflected 

 by a lens or speculum, by which they are prevented from 

 uniting in the same point. It is occasioned by the figure 

 of the glass or speculum, or by the unequal refrangibility 

 of the rays of light. 



Absorp'tion. — The process or act by which light is made pas- 

 sively to disappear in some other substance, through mo- 

 lecular or other invisible means. 



Ac'tinism. — That power in the sun's rays by which chemical 

 changes are produced, as in photography. 



Actin'ograph. — An instrument for registering the variations 

 of the chemical influence of the solar rays. 



Anamorpho'sis, — A distorted representation of an object, so 

 contrived as to appear symmetrical, or an exact representa- 

 tion when seen from a certain point of view, or as reflected 

 by a curved mirror, or through a polyhedron. 



Apparent Magnitude.— The angle under which any line 

 appears at the eye, or the angle made by lines drawn from 

 its extremities to the eye. 



Apparent Motion. — The seeming motion of a body arising 

 from some other cause than its actual motion. 



Astrom'eter (Gr. astron, a star, and metron, a measure"). — An 

 instrument invented and employed by Sir John Herschel 

 for the purpose of comparing the intensities of light of the 

 stars, one with another, by the intervention of the moon, 

 or the planet Jupiter, or some other natural standard. 



Ax'is (Visual). — A particular ray of light from any object, 

 which falls perpendicularly on the eye. 



Catop'trics (Gr. kata, against, and ojJtomai, to see). — That 

 part of optics which explains the properties of reflected 

 light. 



Chromat'ics (Gr. chroma, color). — That part of optics which 

 treats of the properties of the colois of light and of 

 natural bodies. 



Depolariza'tion. — The act of deprivina; of polarity, as the rays 

 of light. 



Di'chroism. — A property of some crystallized bodies of appear- 



