140 PHYSICAL PROPERTIES OF POLARISED LIGHT. 



from the windows of the Luxembourg Palace through 

 a double refracting prism. He observed that when the 

 prism was in one position, the windows with their golden 

 rays were visible ; but that turned round a quarter of a 

 circle from that position, the reflected rays disappeared 

 although the windows were still seen. 



The phenomenon of double refraction was noticed, in 

 the first instance, by Erasmus Bartolin, in Iceland-spar, 

 a crystal the primary form of which is a rhombo- 

 hedron ; who perceived that the two images produced 

 by this body were not in the same physical conditions.* 

 It was also studied by Huyghens and Sir Isaac Newton, 

 and to our countryman we owe the singular idea that a 

 ray of light emerging from such a crystal has sides. 

 This breaking up of the beam of light into two, which 

 is shown by looking through a pin-hole on a card 

 through a crystal of Iceland spar, when two holes be- 

 come visible, is due to the different states of tension in 

 which the different layers constituting the crystal exist. 



In thus separating the ray of light into two rays, the 



* Bartholin, On Iceland Crystals: Copenhagen, 1669. An Accompt 

 of sundry Experiments made and communicated by that Learn d 

 Matliematician Dr. Erasmus Bartholin, upon a Chrystal like 

 Body sent to him out of Island: in connection with which Dr. 

 Matthias Paissenius writes : The observations of the excellent 

 Bartholin upon the Island Chrystal are, indeed, considerable, as 

 well as painful. We have here, also, made some tiyals of it upon 

 a piece he presented me with, which confirm his observations. 

 Mean time he found it somewhat scissile and reducible by a knife 

 into thin laminas or plates, which, when single, shew'd the object 

 single, but laid upon one another shew'd it double ; the two images 

 appearing the more distant from one another, the greater the num- 

 ber was of those thin plates laid on one another. With submission 

 to better judgements I think it to be a kind of Selenites. Some 

 of our curious men here were of opinion that the Rhomboid figure 

 proper to this stone was the cause of the appearances doubled 

 thereby. But having tryed whether in other transparent bodies 

 of the like figure the like would happen, we found no such thing 

 in them, which made us suspect some peculiarity in the very Body 

 of the stone. Phil. Trans, for 1670, vol. v. 



