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XLV^III. Second Vindication of Dr. HerscheVs Theory of 

 Coloured Rings j in Answer to an anonymous Reviewer, 



:. To Mr, TillocL 



Sir, X HE rhembers of the club who, formerly addressed; 

 you, have lately seen, in the twenty-firstNumber of the work 

 called The Retrospect, some strictures on their vii>dication 

 of Dr. Herschel's Essay on the Newtonian concentric 00-^ 

 loured rings, which you honoured with a place in yovir 

 Magazine for November last*. , , 



Our main ohjeci in that communication, was to assert 

 the validity of Dr. Herschel's important e;xperiment de- 

 scribed in the thirty-first article of his Essay. This ex--, 

 periment, the retrospectors, in their thirteenth number, at;- 

 tcmpted to set aside as nugatory^ by affirming that the wedge, 

 of air described by Dr. Herchel was much too thick for' 

 exhibiting the coloured streaks which, according to the 

 Newtonian doctrine of the fits of easy reflection and easy 

 transmission of the rays, would have been seen in it, had 

 it been sufficiently thin. In our former paper we showed^ 

 on the authority of sir Isaac Newton himself, that the re- 

 trospectors had asserted what was not just; and that Dr. 

 Herschel's wedge, according to his measures minutely 

 stated, was sufficiently thin for exhibiting the coloured 

 streaks, if the Newtonian fits had a real existence; and 

 that, according to this hypothesis, the author had a right 

 to expect such coloured streaks ♦ which failijigj or not ap- 

 pearing, he had a right, as he contended for, to conclude 

 that these fits are imaginary. 



One proof concerning the competent thickness of his 

 wedge being so complete, and so much held up to view by 

 appearing in your excellent philosophical miscellany, the 

 retrospectors have thought it necessary, as it would ap- 

 pear, to strike to it, by saying in number twenty-one, page 

 403, '' We now come to the last and the most plausible 

 objection that Dr. Herschel's friends have advanced to our 

 remarks ; and here we grant, that such a wedge as they 

 have described ought to have produced the effi^cts which 

 Dr. Herschel expected from it." Now wc must observe 

 that the wedge of air we described was no other than the 

 wedge described by Dr. Herschel: and what he expected 

 from it, and what every body else must have expected, was 

 this ; namely, coloured sireaks, — provided the Newtonian 



• Philosophical Maj^aziue, vol. xxxiv. p. 359. 

 Vol. 35. No. 144. Jpril 1810. T fitfl 



