^90 Hist07'icai Account of Discoveries respecting the 
that with which the astronomer can compute the motions and 
positions of the heavenly bodies. 
In executing this task, we shall explain the phenomena in the 
order of their discovery. By this means each article will be 
complete in itself, and the reader will be less dependent on the 
popularity and perspicuity of our illustrations. In the adoption, 
therefore, of this plan, the materials may be arranged under five 
periods. 
Period I. Containing the discoveries of Erasmus Bartholinus. 
II. The discoveries of Christopher Huygens. 
III. The investigations, of Newton, Beccaria, Martin, 
Haiiy, A¥ollaston, and La Place. 
IV. The discoveries of Mai us. 
V. The discoveries made after Malus's death, by M. 
Arago, M. Biot, Dr Brewster, Dr Young, M. 
Seebeck, and M. Fresnel. 
Before entering, however, on tiie first of these periods, w^e 
shall endeavour to explain in general what is meant by Double 
Refraction and Polarisation ; for though the explanation of 
these and other terms will necessarily be introduced in describ- 
ing the phenomena to which they have been applied, yet there 
are many of our readers wLo will content themselves with the 
slight knowledge conveyed in a definition, without being at the 
trouble of any farther investigation. 
If a beam of light from the sun or any other luminous 
body is made to pass through a plate of glass or a mass 
of water contained between two parallel plates of glass, it 
will have the same appearance and the same properties af- 
ter transmission as before it. Hence the glass and the water 
refract singly, and all objects seen through them will appear 
single. If the same beam of light is made to pass through a 
parallel plate of calcareous or Iceland spar, or of various other 
crystallised bodies, it will be divided, at its entrance into the 
plate, into two separate pencils or beams. The crystals w’hich 
possess this property are called doubly refracting crystals^ and 
the beam of light is said to be doubly refracted^ and hence all 
objects seen through such crystals will appear double. As one 
of these pencils .or images is refracted according to the ordinary 
