INTERFERENCE OF LIGHT 333 



the slit the light diverges and falls upon the bi-prism, which 

 should for this experiment be about 2 inches square, so as to 

 intercept the whole pencil of light at a convenient distance 

 from the slit. Perpendicular fringes will appear on the screen. 

 Sometimes they appear more plainly by interposing a red glass ; 

 and are made more conspicuous by interposing an achromatic 

 focussing lens between prism and screen, at considerably more 

 than its focal distance from the prism ; the lens then projects 

 the bands in the focal plane of its own conjugate focus. Much 

 depends upon the screen distance, brilliance of the light, 

 and angle and workmanship of the bi-prism. A small angle 

 gives the broadest fringes, and I was only able to obtain 

 visible results with the lime -light by employing a fine prism 

 ground for the purpose by Mr. Ahrens, of very small angle. 

 With this the fringes were 

 amply conspicuous for a good- 

 sized class-room, but would 

 require the arc-light for a 

 large lecture theatre. This is 

 one of the few cases in which 

 an achromatic focussing lens " FlG 188 



gives far superior results, the 



bands being more sharply defined. An achromatic of about 8 

 inches focus and 3| inches diameter is the most suitable for 

 this particular experiment. 



A smaller bi-prism, about an inch square, may be obtained 

 for about 5s., and will project visible fringes with the arc 

 light ; but for the 0. H. jet must be used in another way. It 

 may be mounted in a short cell which fits into the nozzle of 

 the optical objective, and will thus divide the image of any 

 object in the stage into two, which cross or overlap each 

 other. That object in this case is a black card 4 x 2 inches, 

 cut into equal stripes as in fig. 183, or the bright lines may 

 be scraped away on blackened glass. A few slides of different 

 gauges should be prepared, as the best effect is produced by 



