58 PHYSICAL PROPERTIES OF MINERALS, 



DIAPHANEITY. 



Diaphaneity 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 passing from one 

 medium into another of different density : as from air into 

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

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

 pass into water at S, it becomes changed 

 in direction to S U, instead of going 

 straight in its course, R S T. The line 

 a S c is a perpendicular to the surface of 

 the water, and the greater refraction of 

 the water is seen by the bending of the 

 ray toward this perpendicular. If a 

 circle be described about S as a center, 

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

 parallel to the surface of the water, we see by these lines 

 the exact relation between the amount of refraction in these 

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

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



What is diaphaneity ? Explain the terms transparent, Sec. What 

 is meant by refraction 1 Explain from the figure. 



* In mathematical language, U b is the sine of the angle of retrac- 

 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, 



