90 On the MOTION of LIGHT. 



the weft of this interfeflion, and, in twenty-four hours, will 

 defcribe round it a circle, whofe diameter fubtends an angle of 

 lo", but in a diredlion oppofite to that defcribed round the in- 

 terfedtion of the crofs wires of the plane fights. The intelli- 

 gent reader will eafily fee that thefc dedudions are juftly made 

 from the premifes. 



Mr BoscoviCH, in the laft place, (hows that if light be re- 

 tarded in its paflTage from air into water, the appearances with 

 the water telefcope will be diametrically oppofite to thofe above 

 defcribed, and therefore earneftly propofes this experiment to 

 philofophers, as a mean of deciding that iinportant queftion in 

 phyfics. I call it an important queftion ; becaufe the accelera- 

 tion of light in the inverfe proportion of the fines of incidence 

 and refradlion affords an inconteftible proof that the forces 

 which refrad light towards the perpendicular are diredled perpen- 

 dicularly toward the refradling furface, and nearly demonftrates 

 that light confifts of corpufcles emitted by the fhining body. 

 The retardation of light, in the diredl proportion of the fines of 

 incidence and refraction, is totally incompatible with this hy- 

 pothefis concerning the nature of lightj and, in my opinion, 

 with the hypothefis of thofe who maintain that vifion is pro- 

 duced by the undulations of an elaftic fluid, although it has 

 generally been fuppofed to be a confequence of that hypo- 

 thefis. 



T HAVE already faid that my repeated attempts to conftrudl 

 a water-telefcope of fuffi-cient magnifying power ha-ve hitherto 

 failed, in confequence of my not being able to find a fluid fuf- 

 ficiently tranfparent. Lime-water is the moft tranfparent fluid 

 that I know ; and I have filled widi it a telefcope five feet long. 

 But, when I increafed its inagnifying power to more than thirty 

 times, it was vaftly too dark, although the aperture was fo 

 great as to make it very indiftlnd. I am therefore convinced, 

 that although I fliould employ Mr Boscovich's moft beautiful 

 and ingenious conftrucflion to remove the indiftindnefs, there 



would 



