i6o ConjeKures on the Rainbow. [Book 111, 



Porta, of Naples, invented the camera obfcura ; and 

 his experiments upon that inftrument convinced him 

 that light is a iiibftance, by the intromifiion of which 

 into the eye vifion is performed ; for it is proper to 

 mention, that before his, time the opinion was afmofl 

 general, that vifion depended upon what was termed 

 .vi/ual rays> proceeding from the eye. In this the fyf- 

 tem of Porta correlponds nearly with that of" Mauro- 

 lycus : but it ought to be remarked, that the difcove ~ 

 ries of each of thefe two philofophers were unknown to 

 the other. He fhews, moreover, that a defect of light 

 is remedied by the dilatation of the pupil, which con- 

 tracts involuntarily when expofed to a ftrong light, 

 and opens when the light is faint and knguid. 



One Fletcher, of Breflau, in 1571, endeavoured to 

 account for the phenomena of the rainbow, by a dou- 

 ble reflexion and one refraction -, but Antonio de Do- 

 minis, whofe treatife was published in 1611, was the 

 firft who came near to the true theory. He defcribes 

 the progrefs of the ray of light through each drop of 

 the falling rain ; he fhews that it enters the upper part 

 of the drop, where it fuffers one refraction 3 that it is 

 reflected once, and then refracted again, fo as to come 

 directly to the eye of the fpectator j why this refrac- 

 tion 'mould produce the different colours was referved 

 for Sir I. Newton to explain. 



The litter end of the fixteenth century was illuftri- 

 ous for the invention of telefcopes. It is generally 

 allowed to have been cafual. That effect of refrac- 

 tion, which caufes the rays of light, in paffing through 

 a denfe medium thicker in the middle, to converge 

 to a point, and allb that which takes place when they 

 pafs through one thicker at the extremities, had been 

 long obferved j and the afliftance which convex and 

 concave glaffcs afforded to the fight had brought them 



into 



