Intelligence and Miscellanies. 37 1 



when a has moved through another quarter of the circle the 

 hght will be as much reduced as it was when the lines at c 

 coincided. 



Light may be seen through the aperture in either a or 6, 

 when these are looked through separately. But as we ob- 

 serve that there is one relative position of a and 6 when they 

 are together in which the light that has passed through a 

 does not pass through 6, it is evident that it has undergone 

 some change in passing through «, and is no longer ordinary 

 light ; and by examining its new properties it is found to 

 have been polarised by passing through a. 



Now bring hole 2 of the plate d over the hole of the in- 

 strument, and put the instrument together as already direct- 

 ed, and while looking through the aperture at the bottom of 

 &, move fig. 3 gradually round by means of the external pin, 

 and it will be found that the light is alternately restored and 

 extinguished as the pin is moved round. Hence the frag- 

 ment of carbonate of lime, split parallel to its natural filanes, 

 which covers hole 2, is found capable, in certain positions, of 

 altering the property the light has acquired in passing 

 through a, or in the language of most authors on this sub- 

 ject of depolarising it. 



It is not intended here to enter into the theory, or into the 

 details of the phenomena of polarisation further than is ne- 

 cessary for explaining the few terms about to be used, and 

 without which those terms could not be at all intelligible. 

 Transparent bodies may in reference to their action on light 

 be divided into two classes. In owe, the rays of light ivhich 

 pass perpendicularly through them proceed always in single 

 straight lines. These are said to possess only single refrac- 

 tion. 



In the other class, the rays which penetrate the bodies, 

 are divided at their entrance into two portions, which pass 

 in two diverging straight lines through the substance. These 

 portions at their emergence are found to have become po- 

 larised, and to have acquired contrary states of polarisation, 

 so that one portion of each divided ray, possesses properties 

 similar to those of light which has been polarised by reflec- 

 tion, and the other portion has become similar to that light 

 which has been, as before described, polarised by passing 

 through a reflecting surface after falling upon it at the polar- 

 ising angle. 



Substances which possess this property of dividing the 

 rays that pass through them, are said to be doubly refractive, 



