NOTES TO BOOK IX. 50 



accepimus, ct hinc fortasse mensuram illam quse in sinibus 

 consistit elicuerit." Isaac Vossius, De Lucis Naturd et Pro- 

 prietate, says that he also had seen this law in Snell's 

 unpublished optical Treatise. The same writer says, 

 " Quod itque (Cartesius) habet, refractionum momenta 

 non exigenda esse ad angulos sed ad lineas, id tuo Snellio 

 accoptum ferre debuisset, cujus nomen more solito dis- 

 simulavit." 



Huyghens"* assertion that Snell did not attend to the 

 proportion of the sines is very captious ; and becomes 

 absurdly so, when it is made to mean that Snell did not 

 know the law of the sines. It is not denied that Snell 

 knew the true law, or that the true law is the law of the 

 sines. Snell does not use the trigonometrical term sine, 

 but he expresses the law in a geometrical form more 

 simply. Even if he had attended to the law of the sines, 

 he might reasonably have preferred his own way of 

 stating it. 



James Gregory also independently discovered the true 

 law of refraction ; and, in publishing it, states that he had 

 learnt that it had already been published by Descartes. 



I have omitted many interesting parts of the history 

 of Optics about this period, because I was concerned with 

 the inductive discovery of laws, rather than with mathema- 

 tical deductions from such laws when established, or appli- 

 cations of them in the form of instruments. I might 

 otherwise have noticed the discovery of Spectacle Glasses, 

 of the Telescope, of the Microscope, of the Camera Obscura, 

 and the mathematical explanation of these and other phe- 

 nomena, as given by Kepler and others. I might also 

 have noticed the progress of knowledge respecting the Eye 

 and Vision. We have seen that Alhazcn described the 



