INTERFERENCE OF LIGHT 



327 



placed in a row, bubbles 9 or 10 inches diameter can be easily 

 placed upon the rings, and the parallel (or slightly divergent) 

 beam being sent through them all, will produce a very fine 

 effect. 



Flat films produce the best optical phenomena. Taking 

 one up by the apparatus o in fig. 178, the whole arrange- 

 ments are as shown in fig. 179. Here L is the nozzle of an 

 ordinary lantern with the objective on, but the parallel (or 

 rather slightly convergent} beam from the open flange nozzle 

 is better; the rays 

 striking on the soap- 

 film A. The lantern is 

 deflected nearly par- 

 allel with the screen, 

 and the film A stands ' 

 at an angle of 45 

 with the beam, so as 

 to reflect the rays to 

 the screen, on the 

 way to which they 

 are focussed by the 

 loose lens p. 1 Bands 

 of colour speedily ap- 

 pear across the film, & ] *z\ 

 which gradually pass PIG 179 _ plat SoapFilm 

 upwards (the image 



being inverted) as it thins, until at length a yellowish-white, 

 and finally the grey-black appears, after which it soon breaks. 



1 This experiment being typical, the arrangements should be noted. On a 

 table, the pieces of apparatus will be simply arranged separately, standing as 

 figured. With a revolving stand and board such as fig. 99, the holder bearing 

 the film A will stand on the longitudinal board ; and the cross-piece c D (fig. 99) 

 being brought to that spot, the lens p will stand on that at the required 

 distance. This general arrangement governs all experiments where focussing 

 has to be done after the deflection of the rays, by reflection or otherwise, hag 

 been effected. 



