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 porte 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 



