82 GREAT MEN OF SCIENCE 



Thus for example, refraction may take place upon normal 

 incidence. Huygens was able to make all these peculiar 

 phenomena intelligible and predictable by means of his prin- 

 ciple, by the simple assumption that in the crystal, a part of 

 the light is not propagated in all directions with the same 

 velocity, but more slowly in the direction of the crystalline 

 axis, and with increasing velocity in directions at an angle to 

 the axis; fastest of all in a direction at right angles to it. The 

 supposition that light may travel at different velocities in 

 different directions through the crystal was not improbable, 

 since in other respects, for example as regards their mechan- 

 ical strength, crystals generally exhibit the peculiarity of 

 possessing different properties in different directions; this as- 

 sumption also proved to be of great value. Huygens compared 

 the conclusions drawn from his principle of the propagation 

 of light with reality by means of numerous observations on 

 Iceland Spar, and in part also on quartz (rock crystal); in 

 this connection, he also made his important discovery of the 

 polarisation of light. He recognised that rays of light in 

 passing through a first crystal of Iceland Spar are given *a 

 certain form or arrangement, according to which they 

 behave differently, according to the position in which they 

 meet a second crystal of Iceland Spar.' The nature of 

 this certain form was not determined until a hundred and 

 forty-three years later, by Fresnel. We see how far Huygens 

 had progressed in these investigations. They were first 

 published in the year 1676 by a discourse to the Academy 

 in Paris, and then in 1690 under the title Essay on Light ^ one 

 of the most remarkable documents of early great progress 

 in scientific investigation. The last essay published by 

 Huygens himself, called Costnotheoros and dedicated to his 

 brother Constantine, discusses thoroughly the idea previously 

 put forward by Giordano Bruno regarding the multiplicity 

 of planets similar to the earth in space, fit for the habitation 

 of living beings and probably also inhabited. Huygens 



