566 SPECIAL METHODS OF DISCOVERT. 



through such a crystal. In the year 1669 he published a 

 book containing an account of his observations on the 

 subject, stating also that he had noticed that one of the 

 two images was produced by the ordinary mode of refrac- 

 tion, and the other by an extraordinary or new rule, and 

 varied in different positions. Huyghens also discovered, 

 by means of observation, the fact of polarisation of light 

 by crystals. The discovery of polarisation of light by 

 reflection, was first observed by Malus, and soon led by 

 means of additional observations to that of polarisation 

 by refraction, the plane of polarisation being perpendicular 

 to that of reflection. He also found that when a portion of 

 a beam of light was polarised by reflection, a corresponding 

 portion was polarised by transmission and refraction, the 

 planes of the two polarisations being at right angles to each 

 other ; and that the polarisation, either by reflection or 

 refraction, which was only small in amount when a single 

 glass plate was employed, might be made more and more 

 complete by increasing the number of plates. It was by 

 means of observation that John Fabricius discovered the 

 solar spots, and by regular and systematic observations of 

 those spots, during a period, first of 12 (and ultimately 

 of 34) years, that Schwabe of Dessau, discovered a regular 

 decrease of their number during 5J years, and then an 

 increase during another 5J years. De Luc, in the year 

 1755, observed that ice, during the act of melting, did 

 not rise in temperature above the freezing-point until it 

 was entirely melted ; and Dr. Black confirmed that obser- 

 vation. By means of observations made in balloons, the 

 fact was discovered, that the temperature of the atmo- 

 sphere decreases regularly up to a certain point (3,000 to 

 6,500 feet high) during ascent, and varied on different 

 days ; the decrease was then stationary for two or three 

 thousand feet, and this was the region of clouds; and 



