72 GREAT MEN OF SCIENCE 



these clocks, such as we still find to-day in the escapements 

 of pocket watches and ships' chronometers. Here again, 

 only the introduction of this essential feature of accurate 

 running was the point of novelty. Pocket watches with 

 trains of wheels, driven by a spring, existed already; they 

 were known, on account of their external form, and their 

 main place of origin, as 'Nuremberg eggs.' In the matter of 

 the introduction of the spring controlled balance wheel, in 

 place of the gravity pendulum, Hooke in England, where the 

 need of good ships' chronometers was most pressing, had 

 priority over Huygens, but not before the latter had realised 

 and introduced the ordinary pendulum clock, the model for 

 the construction of the chronometer. 



Huygens further strove after extreme refinement to an 

 astonishing degree in his pendulum clocks. His profound 

 investigation of the peculiarities of the cycloid, which was in 

 other respects of great value, led to his invention of the 

 cycloidal pendulum, in order to produce constancy of period, 

 even in the case of a large arc of swing. Though we have 

 again abandoned the special arrangements fully described 

 by Huygens, which are necessary for the use of this type of 

 pendulum, it is because we have learned to work with very 

 small arcs of swing; nevertheless, the Horologium Oscilla- 

 torium arouses the reader's admiration for the 'double 

 virtuosity' with which Huygens carried out, in the most 

 perfect manner, what had to be derived both from mechanical 

 practice, and from the most subtle geometrical synthesis. 



About the same time - from 1652 onwards - Huygens was 

 also busy on improvements in the telescope; upon learning of 

 Snell's law of the refraction of light, which was not yet known 

 to Kepler and Galileo, he was able to estimate the effects of 

 lenses with much greater accuracy than had hitherto been 

 possible. He thus arrived also at an understanding of the 

 errors of lenses (spherical and chromatic aberration) and their 

 dependence upon focal length and aperture. He also ground 



