LIGHT. 93 



ing under two distinct colors, according to the direction in 

 which they are viewed, or in which the rays of light pass 

 through them. 



Diffrac'tion. — A change which light undergoes, when, by pass- 

 ing very near the borders of an opaque body, it forms 

 parallel bands or fringes. 



Diop'tries (Gr. Jioptumai, to see through). — That part of 

 optics which treats of the refractions of light passing 

 through different mediums, as through air, water, or glass. 



Emis'sion. — The theory, adopted by Newton, that light consists 

 of exceedingly minute particles of a peculiar matter thrown 

 out by a luminous body. 



Fo'cus. — The point of convergence at which all the I'ays of 

 light meet after passing through a convex lens. 



Irradia'tion. — An apparent enlargement of objects beyond 

 their proper bounds, in consequence of the vivid impression 

 of light on the eye. 



Optom'eter (Gr. dps, sight, and metron, a measure). — An in- 

 strument for- measuring the limits of direct vision. 



Photog'raphy (Gr. jihos, light, and grapho, I write). — The 

 science of the action of light on bodies. 



Photd'ogy (Gr. pliOs, light, and logos, a discourse). — The doc- 

 trine or science of light, explaining its nature and phe- 

 nomena. 



Photom'eter (Gr. j^ltos, light, and metron, a measure). — An in- 

 strument for measuring the relative intensities of light. 



Photom'etry (Gr. pAos, light, and metreo, I measure). — The 

 science which treats of the measurement of light. 



Pleoch'roism. — The exhibition of several shades of colors, as 

 when a mixture of polarized and non-polarized light passes 

 through a double refracting crystal. 



Polarim'etry. — The art or process of measuring the polariza- 

 tion of light. 



Ra'diant. — The luminous point or object from which light 

 emanates. 



Reflec'tion. — The rebound or turning back of rays of light 

 from a smooth opposing surface. 



Refrac'tion. — The deviation of a ray of light from a right line 

 in entering a medium of greater den.^ity. 



Refrangibil'ity. — A disposition of rays of light to be refracted 

 or turned out of a direct course, in passing out of one 

 transparent body or medium into another. 



