26 IOWA ACADEMY OF SCIENCES. 



In 1665 Robert Hooke suggested a wave theory of light, and 

 suoh a theory was elaborated by Huygens in a paper before the 

 Piench Academy in 1678. He assumed the existence of an all 

 pervading ether. He described the double refraction in Ice- 

 land spir and observed that both rays were polarized. But 

 polarization received little further attention until the time of 

 Young and Fr :'sael ia our century. The wave theory met the 

 determined opposition of Newton, whose great and increasing 

 authority caused it to sink out of sight for more than 100 years, 

 when it was again brought into prominence by Young in 1801, 

 and was finally established by the experiments of Foucault and 

 F.zeau on the velocity of light in 1850. Though a supporter of 

 the corpuscular theory of light chiefly because it explained the 

 propagation of light in straight lines, Newton made several 

 important additions to our knowledge of ligh% and probably 

 failed to discover the spectroscope only because the beam of 

 light that fell upon his prism came through a round hole 

 instead of a slot parallel to the edge of his prism. He was the 

 first to explain disp-rsion upon difference of refrangibility of 

 the rays. He believed that it was not possible to make an 

 achromatic lens and, therefore turned his attention to a reflect- 

 ing telescope which he invented in 1668. 



The phenomenon of diffraction was discovered by Grimaldi 

 in 1666 and experimentej upon by Newton, but like polariza- 

 tion it had to wait for its explanation until 1815, when Fresnel 

 r discovered the phenomenon. The eighteenth century wit- 

 nessed little progress in light beyond fie construction of 

 achromaitic leases by DoUand in 1758, and their application to 

 the telescope and microscope. 



HEAT. 



The first thermometer was invented by Jean Rey, in 1632, by 

 inverting the thermoscope of Galileo and filling the bulb and 

 part of the stem with water. Twenty-five years later the end 

 of the bulb was sealed, and alcohol replaced the water. Mer- 

 cury was first used in 1659. The thermometer was perfected 

 and the present fixed points adopted by Fahrenheit in 1724, 

 and Celsius in 1742. The discovery that liquids have definite 

 boiling points is apparently due to the former. 



The ideas of specific heat and latent heat, apparently origi- 

 nated with Joseph Black in 1756, and he determined the latent 

 heat of vaporization of water and liquifaction of ice. The 



