266 On {he Affections of Light 



These facts admit, to a certain extent, of an easy explanation, 

 if we suppose that the pkite of agate consists of' laminae mn, op 

 imperfectly transparent, alternating with laminae cd, ef, which 

 are more pervious to light, a structurr which is indicated by the 

 existence of a bright and a nebulous image. In this case the 

 intensity of the light will obviously be a maximum when the ray 

 RS is parallel to mn, and a ray PQ will, within certain limits, 

 suffer less diminution of lustre than a ray TV falling with the 

 same angle of incidence on the other side of RS. 



When RS and mn are per])cndicular to the surfaces of AB, 

 and when PQ and TV form equal angles with the perpendicular 

 RS, their intensity should be equal ; but this is by no means 

 the case, for the transmitted light which is incident on the side 

 T of RS appears to have a different character from that which 

 is incident on the other side. V/e must therefore suppose that 

 there is some other peculiarity of structure in the agate, con- 

 nected probably with that particular kind of crystallization which 

 polarizes light, to which tliis curious fact must be ascribed*. 



The intensity of the light transmitted by the agate is likewise 

 affected by its polarizing property. If a ray Rr, Plate IV,, fig. 4, 

 is incident upon a piece of agate AB, so as to be polarized by 

 reflection from the second surface, then, since it is polarized 

 during its passage from r to E, the bright image will suffer total 

 reflection at E, while the nebulous image will be transmitted 

 like common light in the direction EG. If the agate is now 

 turned round 90°, the nebulous image will suffer total reflection 

 at E, while the bright image will penetrate the second surface 

 at E like common light. ^Vhen the incident ray Rr has dif- 

 ferent obliquities and the agate intermediate positions, the in- 

 tensity of the transmitted light will be more or less affected by 

 its polarizing power. 



The preceding observations on the laminated structure of tlie 

 agate enable us to give a satisfactory explanation of some sin- 

 gular appearances exhibited bv that mineral. 



In the specimen' shown in Plate IV. fig. 5, the black lines re- 

 present the veins, and consequently the direction of the laminae, 

 and the dotted lines ah, ac, cm, ck, &;c. are drawn through the 

 vertices of the angles made by the veins ; and consequently by 

 the laminae whenever they change their direction. When light 

 is transmitted through a piece of agate of this description, the 

 planes Aacm, mck, ngf, nkg have the appearance of being dif- 

 ferently inclined to one another, and transmit different quan- 

 tities of light. If the veins and the laminae preserved the same 

 inclination to the surface of the plate of agate when they changed 



* See Edinburgh Transactions, vol. vii. part ii. 



their 



