6 



Upper figure a nearly equal- sided fragment, to show the 

 nucleus and the double refracting property, by being placed 

 on a straight line, which appears displaced and doubled when 

 viewed through the upper opposite face. The sides only re- 

 flect the object, for we cannot see a figure through the edges 

 of the crystal. 



Middle figures the same, somewhat thinner, placed on 

 letters to show that the refraction divides towards the obtuse 

 angles. 



Lozver figure, a rare fragment of a fine yellow. The 

 prismatic colours caused by the flaws are in the regular 

 order of the rainbow : the brightness depends on the polish 

 of the surfaces, and the closeness of the flaw, nearness to 

 the surface, &c. The upper lighter fracture is paler, because 

 the more open it is the less visible the colours. The 

 opaque white at the eds;e is in consequence of a blow in a 

 direction contrary to the laminse, which always bruises 

 it. Please to read ioi° 30 y , 7 8° 30', instead of what is 

 engraved. It was thought proper to picture one face of 

 each of these figures as near as might be to the measured 

 angles* (the largest side of each). It may be fairly ob- 

 served that, according to the rules of perspective, the other 

 sides could not be seen ; some rules may give way to per- 

 spicuity occasionally. Small objects cannot conveniently 

 be observed by this rule, and very few people look perspec- 

 tively, as it requires great nicety. These figures are intended 

 to give a natural appearance to a general observer, who 

 looking with two points of sight sees more of the subject. 

 It is intended to use one point of sight where it should be 

 found most eligible. 



We must beg leave to refer our readers to Dr. W. H. 

 Wollaston's learned paper on the oblique refraction of the 

 Iceland crystal (Phil. Trans, for 1802, part 2, p. 381), 

 for an account of its refracting property. 



* A fragment placed by the angles will be found nearly to correspond 

 •with. diem. 



