LIGHT. 137 



front of the condensers that carries a series of plain 

 glass plates inclined to the beam so that it meets it at 

 the polarizing angle of glass. Part of the light is trans- 

 mitted and is absorbed by a piece of black cloth. The 

 light that is reflected is sufficiently well-polarized for 

 all purposes of demonstration ; and such a beam may 

 be treated in every way like the beam from the porte 

 lumiere and with like results. 



DIFFRACTION. 



Reflect the beam from the parte lumiere through a 

 slit like one for showing the Fraunhofer lines. It 

 ought not to be more than one sixteenth of an inch 

 wide. Receive this beam, without magnifying it, upon 

 a second slit in a screen at a distance of four or five 

 feet from the first slit. Make the room as dark as 

 possible, and then hold a sheet of white paper behind 

 the second slit anywhere from a few inches to several 

 feet. Colored fringes will appear on each side of the 

 central line, with a series of alternate black and white 

 bands or lines. These may be received upon a screen 

 twenty feet away, when they should have a united 

 breadth of a foot or more, but the light is necessarily 

 very weak. A lens does not improve them very much. 



With a piece of perforated paper or tin or lace, or 

 still better, with an eidotrope, which consists of two 

 disks of perforated tin made to revolve in opposite di- 

 rections, like the chromatrope, a very beautiful exhi- 

 bition of the phenomenon of diffraction may be given 

 in the following way : — 



Take two large, short, focus lenses, such as form the 

 condensers in Marcy's sciopticon. Place one close to 

 the opening to the porte lumiere, as shown in the figure. 

 The second one may be put so far in front of the other 



