176 DISSERTATION SECOND. [parti. 



satirist would not deserve much attention, if it were not 

 confirmed by more sober testimony. Pliny, speaking of 

 rock crystal, 1 says, that a globe or ball of that substance 

 was sometimes used by the physicians for collecting the 

 rays of the sun, in order to perform the operation of caute- 

 ry. In another passage, he mentions the power of a glass 

 globe filled with water, to produce a strong heat when ex- 

 posed to the rays of the sun, and expresses his surprise 

 that the water itself should all the while remain quite cold. 

 With respect to the power of glasses to magnify objects 

 seen through them, or to render such objects more distinct, 

 the ancients appear to have observed ill, and to have reason- 

 ed worse. " Literae quamvis minutae et obscurae per vi- 

 trearn pilarn aqua plenam majores clarioresque cernuntur. 

 Siiera ampiiora per nubem adspicienti videntur : quia acies 

 nostra in humido labitur, nee apprehendere quod vult 

 Jideliter potest." 2 This passage, and the speculations con- 

 cerning the rainbow in the same place, when they are con- 

 sidered as containing the opinions of some of the most able 

 and best informed men of antiquity, must be admitted to 

 mark, in a very striking manner, the infancy of the physical 

 sciences. 



2. From Alhazen to Kepler. 



An interval of nearly a thousand years divided Ptolemy 

 from Alhazen, who, in the history of optical discovery, ap- 

 pears as his immediate successor. This ingenious Ara- 

 bian lived in the eleventh century, and his merit can be 

 more fairly, and will be more highly appreciated, now that 

 the work of his predecessor has become known. The mer- 



1 Hist. Nat. Lib. 37, cap. 10. 

 2 Seneca, Nat. Quest. Lib. i. cap. 6. 



