On the Refraction of Light, 353 



and a white surface placed at some distance behind it, each puncture 

 will be seen to contain a perfect image of the flame. 



The small circular aperture displays some singular appearances, 

 when allowed to transmit dispersed light. Thus, place in a sun 

 beam, admitted through a hole in a dark chamber, a spherical mir- 

 ror* of about four lines in diameter, and near it, place in the disper- 

 sed rays, a leaden plate containing a few punctures. At a conven- 

 ient distance receive on a white surface the shadow of the plate, and 

 the following phenomena will be evident: each puncture will contain 

 a number of dark circular lines or epicycles, with luminous intervals 

 between them. On minute examination these lines will be found 

 chromatic, being bordered on the one side by orange, and on the 

 other by bluish light. When the holes in the lead are square or 

 rhombic, the images received on paper will be found to comprise 

 many smaller squares or rhombs. The development of dark and 

 bright lines is not occasioned by a property, exclusively belonging to 

 heterogeneous light; for wten we admit into the dark chamber a 

 beam through red or blue glass, and adjust in it the spherical mir- 

 ror and punctured plate, the • same dark and bright circles are ob- 

 served;, but instead of the chromatic fringes that border the circles 

 in compound light, we shall only perceive dilutions and concentra- 

 tions of the color employed. 



The homogeneous rays emerging from a prism, being transmitted 

 through holes in the leaden plate, present some appearances well 

 worthy of notice. Having admitted a beam of white light into a dark 

 chamber, through a hole half an inch wide, I ordered a prism in such 

 a manner as to decompose the light, and placed in the emerging 

 rays, at some distance from the prism, a plate of lead, having in it a 

 hole of one hne in diameter. At a distance of four- feet from the 

 lead, I held a sheet of white paper, expecting to find on the paper 

 the base of a luminous cone, such as would be formed by white light 

 under the same circumstances ; instead of which, there appeared 

 from the lead to the white surface a pyramid of light whose base 

 on the paper described an oblong figure, bounded on all sides by 

 straight lines. On varying the position of the prism, so as to let the 

 refracted rays emerge at diflerent angles from the incident rays, the 



* This experiment ought to be pei Ibnucd witli a metallic leflcctor. The bulb ol 

 a thermometer answers the purpose, but not so well as metal, since tlic light is liixblc 

 to be decomposed by the slightest Incqualily of the glass. 



