90 On the MOTION of LI G H T. 



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

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

 10", but in a direction oppofite to that defcribed round the in- 

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

 gent reader will eafily fee that thefe deductions are juftly made 

 from the premifes. 



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

 tarded in its paffage 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 important 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 refraction affords an inconteftible proof that the forces 

 which refract light towards the perpendicular are directed perpen- 

 dicularly toward the refracting furface, and nearly demonftrates 

 that light confifts of corpufcles emitted by the mining body. 

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

 incidence and refraction, is totally incompatible with this hy- 

 pothefis concerning the nature of light, 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. 



I have already faid that my repeated attempts to conftruct 

 a water-telefcope of fufficient magnifying power have 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 haA^e filled wish, it a telefcope five feet long. 

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

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

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

 that although I fhould employ Mr Boscovich's moft beautiful 

 and ingenious construction to remove the indiftinctnefs, there 



would 



