66 



Immersion Lenses and New Refraciomders. 



Ptolemy observed that the refractions from water into glass 

 were less than any he had previously observed. The following were 

 some of the results he obtained : — 



Claudius Ptolemy's Tables. 



Refractions feom Glass into Am. 



Angle of Incidence, 

 o 

 

 20 

 60 

 80 



Angles of Refraction. 





 13 

 34 



42 





 30 

 30 







Refeactions from Water into Glass. 



Angle of Incidence. 



o 





 20 



60 

 80 



Angles of Refraction. 



The first Table on the left, for the 4 angles, gives the value of 

 the refractive index for Ptolemy's " pure glass "at 1 * 48, 1 • 53, 

 1*47; the mean of which is 1"49, or nearly the refractive index 

 for our common glass, and evidences the singular care Ptolemy must 

 have taken in his experiments. 



For the water refractions the values of the index of refraction 

 between glass and water are (by the refractometer, to save the cal- 

 culation by logarithms), for 60" and 80° in the Table, nearly 1 '500 

 and 1*336, a surprising approximation to the true values by this 

 primitive method. 



Ptolemy's Immersion Lens. 



Refraction from Water into Glass. 



If the glass cylinder be removed, then the ray would be turned 

 back by the surface of the water, and internally reflected, instead 

 of being refracted. 



