366 Dr. Hersckel's Revieiter reviewed. 



too thick cannot surely be admitted on the reviewer's ipse 

 dixit. This position ought to have been estabhshed by 

 some proof, not by naked assertion ; since the author, in 

 his paper, had so minutely shown that ihe thickness of his 

 \vcdgLS was such as to warrant his concUisions. The truth 

 is, that the reviewer here contradicts not Doctor Herschel, 

 hut Sir Isaac Newton, who has proved iliat plates of air 

 between his object-glasses, as thick, and a great deal thicker 

 than many to be found in Doctor Herschel's wedges, will 

 produce observable changes in uhite light. 



Newton, in his Opticks, edition 1721, page 178, de- 

 termines experimentally the thickness, in parts of an inch, 

 of his plates of air at the first, second, third, fourth, 

 fifth &c. ring, as respectively equal to a fraction whose 

 constint denominator is 178000, and the numerator ac- 

 cording to the series 1.3..'j.7.y, &c. These were the thick- 

 nesses of the respective plates of air between his object- 

 glasses, when his eve was placed perpendicularly in the 

 axes of the rings. Further, in page 179? he determines all 

 those different thicknesses when he viewed the rings 

 obliquely. By his table, given us in that page, it appears 

 that any individual ring of a given diameter, with a certain 

 thickness of air at that place, will, when seen in the most 

 oblique position, be incrtased 3'5 times in diameter, and 

 that the thickness of the plate of air at the same place will 

 be increased \'2''25 times. 



As to the number of rings vhich can be perceived and 

 counted, Newton mentions that, under favourable circum- 

 stances, he has sometimes seen more than twenty of them ; 

 and in the usual vt^av eight or nine. But even supposing 

 we could see none beyond the limits of the eighth ring, the 

 question before us is, what would be the thickness, ac- 

 cording to Newton, of the plate of air at that place ? This, 

 when the eighth ring is seen perpendicularly, with the ob- 



15 

 iect-cksscs which Newton used, will be ...;;:r-^ according 

 * ° 1 78000 ° 



to the foregoing progression of the fractional numerators ; 



, , , ,. , 15 X 12 25 , 18375 , 



and when seen obliquely _^^-^^^ equal to ^^^^^^^ equal 



to parts of an inch. Here tlien we have the authority 



yog "^ ^ 



of Sir Isaac Newton for saying that a ring will be formed 

 and perceivable when light "passes through a plate of air of 



— . The reviewer will not deny that the production of a 



ring is a change made on wliile light. . 



Now 



