378 HISTORY OF OPTICS. 



Vitello the effect of refraction at the two surfaces 

 of a glass globe is clearly traced. 



Notwithstanding Alhazen's assertion of the con- 

 trary, the opinion was still current among mathe- 

 maticians that the angle of refraction was pro- 

 portional to the angle of incidence. But when 

 Kepler's attention was drawn to the subject, he saw 

 that this was plainly inconsistent with the obser- 

 vations of Vitello for large angles; and he convinced 

 himself by his own experiments that the law was 

 something different from the one commonly sup- 

 posed. The discovery of the true law excited in 

 him an eager curiosity; and this point had the 

 more interest for him in consequence of the intro- 

 duction of a correction for atmospheric refraction 

 into astronomical calculations by Tycho, and of the 

 invention of the telescope. In his Supplement 

 to Vitello, published in 1604, Kepler attempts to 

 reduce to a rule the measured quantities of refrac- 

 tion. The reader who recollects what we have 

 already narrated, the manner in which Kepler at- 

 tempted to reduce to law the astronomical observa- 

 tions of Tycho, devising an almost endless variety 

 of possible formulae, tracing their consequences with 

 undaunted industry, and relating, with a vivacious 

 garrulity, his disappointments and his hopes, will 

 not be surprized to find that he proceeded in the 

 same manner with regard to the Tables of Ob- 

 served Refractions. He tried a variety of construc- 

 tions by triangles, conic sections, &c., without being 



