A reply to Mr. Robin's remarks on the Essay upon distinct and indistinct vision published at the end of Dr. Smith's Complete system of optiks / by James Jurin.

  • Jurin, James, 1684-1750.
Date:
1739
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Licence: Public Domain Mark

Credit: A reply to Mr. Robin's remarks on the Essay upon distinct and indistinct vision published at the end of Dr. Smith's Complete system of optiks / by James Jurin. Source: Wellcome Collection.

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    [4 ] and 78, this ingenuous writer filently pa fifes over, and fays / confess at art. 220. that the den- fity of the light is different in diferent -parts of thefe circles. This word confefs I have obferved to be , very familiar with this Gentleman and his affo- dates, though they feem not to comprehend the meaning of it. They make no difference be¬ tween a declaration made by way of caution to the Reader, before any controverfy arifes, and a confeffion extorted by the force of an adverfa- ry’s reafons. In art. 50, 51, 52. where I fpeak of the mea- fure of the Moon’s diameter, as taken by the ancient Aftronomers, and by the moderns ] ike- wife who obferved by plain fights, as the noble Hevelius, I treat of no other advantage they had above other men, but their practice in ob- fervation and the flatnefs of their eyes by age. But in the laft of thofe articles I gave notice, that I fhonld anon confider the cafe of Hevelius in particular, and when I came to do fo in art. 181, &c. I took into confederation the advan¬ tage arifmg from fights. However, I am ready to own, that it was an omillion not to confider the fights made ufe of by the ancients, at the fame time that I was fpeaking of the advantage they might have from praftice and the fiatnefs of their eyes by age. I have fhown in art. 90, 91, 92. that when a white circle bounded by a black circular line as its circumference, is feen by indiftinct Vifion, the circumference appears broad and faint, and its penumbra fpreading inwards, the white circle within it is thereby rendred too narrow. And when the diameter of the circle is fmall enough to fuffer the penumbras from oppofite parts of