58 PHYSICAL PROPERTIES OP MINERALS 



DIAPHANEITY. 



Diapkaneity is the property which many objects possess 

 of transmitting light ; or in other words, of permitting more 

 or less light to pass through them. This property is often 

 called transparency, but transparency is properly one of the 

 degrees of diaphaneity. The following terms are used to 

 express the different degrees of this property : 



Transparent : a mineral is said to be transparent when 

 the outlines of objects, viewed through it, are distinct. Ex. 

 glass, crystals of quartz. 



Subtransparent, or semitransparent : when objects are seen 

 but their outlines are indistinct. 



Translucent : when light is transmitted, but objects are not 

 seen. Loaf sugar is a good example ; also Carrara marble. 



Subtranslucent : when merely the edges transmit light 

 faintly. When no light is transmitted, the mineral is de- 

 scribed as opaque. 



REFRACTION AND POLARIZATION 



Light is always bent out of its course on parsing from one 



medium into another of different density : As from air into 



water, or from water into air. This beranng of the rays of 



light is called refraction. Thus if ayfay of light, as R S, 



92 pass into water eM; S, it becomes changed 



""""""N. in direction torS U, instead of going 



\ straight in its*' course, R S T. The line 



\ a S c is a perpendicular to the surface of 



-j-j the water, and the greater refraction of 



j^gg the water is seen by the bending of the 

 ^gjjj ray /toward this perpendicular. If a 

 p^^ss^l^^y circle be described about S as a center, 

 and the lines R a and'TJ b be drawn perpendicular to a c, or 

 parallel to the surfa/e of the water, we see by these lines 

 the exact relation between the amount of refraction in these 

 two cases ; for the/refraction in water is as much greater than 

 in air as U b isiess than R a.* This relation is called the 



What is diaphaneity ? Explain the terms transparent, &c. What 

 is meant by refraction ? Explain from the figure. 



* la mathematical language, U b is the sine of the angle of reirac- 

 tion, and a R the sine of the angle a S R, the angle of incidence ; the 

 ratio between the two sines is constant, it being alike for every angle of 

 incidence. 



