194 DISSERTATION SECOND. [parti. 



Though the Opticks of Descartes had been published 

 twenty-five years, Gregory had not heard of the discovery 

 of the law of refraction, and had found it out only by his 

 own efforts ; — happy in being able, by the fertility of his 

 genius, to supply the defects of an insulated and remote 

 situation. 



A course of lecturers on opticks, delivered at Cambridge 

 in 1668, by Dr. Barrow, and published in the year follow- 

 ing, treated of all the more difficult questions which had 

 occurred in that state of the science, with the acuteness 

 and depth which are found in all the writings of that geo- 

 meter. This work contains some new views in opticks, 

 and a great deal of profound mathematical discussion. 



About this time Grimaldi, a learned Jesuit, the compa- 

 nion of Riccioli in his astronomical labours, made known 

 some optical phenomena which had hitherto escaped ob- 

 servation. They respected the action of bodies on light, 

 and when compared with reflection and refraction, might 

 be called, in the language of Bacon's philosophy, crepus- 

 cular instances, indicating an action of the same kind, but 

 much weaker and less perceptible. Having stretched a 

 hair across a sunbeam, admitted through a hole in the 

 window-shutter of a dark chamber, he was surprised to find 

 the shadow much larger than the natural divergence of the 

 rays could have led him to expect. Other facts of the 

 same kind made known the general law of the diffraction 

 or inflexion of light, and showed that the rays are acted on 

 by bodies, and turned out of their rectilineal course, even 

 when not in contact, but at a measurable distance from the 

 surfaces or edges of such bodies. Grimaldi gave an ac- 

 count of those facts in a treatise printed at Bologna in 

 3665. ' 



1 Physico-Mathesis de Lumine, Coloribus, &c. in 4 to. 



