
Nor is it material whether the Hair be en compassed with Air, or with any other pellucid Substance. And parti cularly a Hair of a Man's Head, whose breadth was but the 280th part of an Inch, being held in this Light, at the distance of about twelve Feet from the Hole, did cast a Shadow which at the distance of four Inches from the Hair was the sixtieth part of an Inch broad, that is, above four times broader than the Hair, and at the distance of two Feet from the Hair was a bout the eight and twentieth part of an Inch broad, that is, ten times broader than the Hair, and at the distance of ten Feet was the eighth part of an Inch broad, that is 35 times broader. Through this Hole I let into my darken'd Chamber a beam of the Sun's Light, and found that the Shadows of Hairs, Thred, Pins, Straws, and such like slender Substances placed in this beam of Light, were considerably broader than they ought to be, if the Rays of Light passed on by these Bodies in right Lines. I made in a piece of Lead a small Hole with a Pin, whose breadth was the 42d part of an Inch, For 21 of those Pins laid to gether took up the breadth of half an Inch.

For the circumstances of the Phænomenon, so far as I have observed them, are as follows. These broad Shadows and Fringes have been reckon'd by some to proceed from the ordinary refra ction of the Air, but without due examination of the Matter. But if the Hole be enlarged the Fringes grow broad and run into one ano ther, so that they cannot be distinguish'd. GRIMALDO has inform'd us, that if a beam of the Sun's Light be let in to a dark Room through a very small hole, the Shadows of things in this Light will be larger than they ought to be if the Rays went on by the Bodies in strait Lines, and that these Shadows have three parallel Fringes, Bands or Ranks of colour'd Light ad jacent to them. Observations concerning the Inflexions of the Rays of Light, and the Colours made thereby. Download NATP00051.xml and schema (advanced users only).



Source: Opticks: Or, A Treatise of the Reflections, Refractions, Inflexions and Colours of Light.
