INTERFERENCE OF LIGHT 337 



feather stitched with black thread on a blackened card, can 

 be projected in the same way. The colour of the peacock 

 eye will change as its plane is altered ; but as the colour is 

 uniform and not in spectra, in this case the colour must be 

 produced by a thin film. (I believe a portion of the pearl 

 colour to be of the same character.) The plane is readily 

 shifted by turning the pillar a little ; and the object can 

 be readily rotated in its own plane by turning the tablet 

 round in its socket. 



193. Perforated Plates, Perforated zinc will give interest- 

 ing diffraction phenomena if the light is brilliant. Every 

 gauge procurable should be purchased ; then discs should be 

 cut out and mounted in 4 x 2 wooden frames, for the optical 

 stage, and spring wires bent circularly will keep them in 

 place. Blackened perforated cards will answer, but the heat is 

 apt to ignite them. Placing one in the stage, and focussing 

 it rather beyond the screen, the interference of the various 

 small pencils of light passing through the apertures in the 

 card will produce coloured effects, rather brilliant a few feet 

 from the nozzle, where they may be shown by holding a 

 sheet of cardboard. Different gauges should be tried for the 

 best results. But still better are produced if another frame 

 and plate be introduced, at a distance varying from J inch to 

 some inches in front of the other, and either the same or 

 some other gauge, which experiment must determine; for 

 every difference in the power of the radiant, and in the screen 

 distance, will determine gauges and distance between the 

 plates which give the best effect. Sometimes a piece of gauze 

 as the front plate will give a good result ; as a rule, the finer 

 gauges of zinc or card give the best, and the coarser of two 

 different plates should be the posterior one. A little adjust- 

 ment of plates and focussing will produce beautiful coloured 

 patterns on the screen, the pencils from the first set of 

 holes being diffracted by the second. If one of the plates 

 is mounted in a rotating frame like fig. 195 (p. 348), and 



