546 PHILOSOPHICAL TRANSACTIONS. [aNNO I67I. 



thinner piece be nimbly broken asunder with your finger. But if you strike it 

 with a hammer, the percussion has not the same effect, nor is equally resisted 

 from every part ; whence the smooth sides of this mineral become often sca- 

 brous. The whole body is rather clear than bright, of the colour of limpid 

 water ; but that colour, when it has been immersed in water and dried again, 

 becomes dull. Hence it is, that in its native place the upper surface is darkish; 

 because of the rains and snows fallen upon it. Sometimes there appear also 

 some reflections of colours, as in the rainbow. The angles are not pointed 

 alike, all the flat sides being obliquely inclined to one another. The opposite 

 plains are parallel. 



In this crystalline prism, two of the plain angles are always acute, and the 

 two other obtuse ; and never any of them is equal to the collateral angles of the 

 inclinations. 



The objects seen through it, appear sometimes, and in certain positions of 

 the prism, double : where it is to be noted, that the distance between the two 

 images is greater or less, according to the diff'erent size of the prism, so that 

 in thinner pieces this difference of the double image almost vanishes. 



The object appearing double, both images appear with a fainter colour, and 

 sometimes one part of the same species is obscurer than the other. 



To an attentive eye, one of these images will appear higher than the other. 



In a certain position the image of an object, seen through this body, appears 

 but single, as through any other transparent body. 



We have also found a position, wherein the object appears sixfold. 



If any of the obtuse angles of this prism be divided into two equal parts by 

 a line, and the visual rays pass from the eye to the object through that line, 

 or its parallel, both images will meet in that line, or in another parallel 

 to it. 



Whereas objects, seen through diaphanous bodies, are wont to remain con- 

 stantly in the same place, in what manner soever the transparent body be moved, 

 nor the image on the surface move except the object be moved ; we have ob- 

 served here, that one of the images is moveable, the other remaining fixed ; 

 although there be a way also to make the fixed image moveable, and the 

 moveable fixed in the same crystal ; and another, to make both moveable. 



The moveable image does not move at random, but always about the fixed, 

 which while it turns about, it never describes a perfect circle, but in one case. 



Dioptrics teach, that a diaphanous body, having one only surface, sends 

 from one object but one image refracted to the eye ; and having more surfaces 

 than one, it represents one image in each ; but whereas in our substance there 

 occurs but one plain superficies to the eye, and yet a double image of one ob- 



