the Black Spot in Liquid Films. 521 



distinguished. After this for quite an interval no image 

 could be seen. At 57°*5 the image was very distinct. The 

 rise in the intensity was remarkably rapid beyond the 

 polarizing-angle. 



The effect of using different values for the mean wave-length 

 in the computed curves is illustrated in fig. 6 by the dotted 



Fig. 7. 

 oo4t 



ete 



ooz 



OOl 



Volanzt ? per/Oi nJzcula r 6?~zxc 'fte ofmciae/zfr ? 



Ca?cu.t 



line, which is the computed intensity-curve for D equal to 

 6 fifJL on the assumption that X is 500 fijA. 



Comparison of the two Methods. — The first method offers 

 many advantages over the second. The films could be seen, 

 the temperature of their atmosphere measured, and its hygro- 

 metric condition more easily controlled in the first method. 

 But it was not possible to have the films thin at the same 

 rate ; hence the readings represent only mean thicknesses. 

 In many cases there was, no doubt, a wide difference in 

 thickness, at any elevation, on different films, and even on the 

 same film. 



The film in the second method was 2 cms. by 9 cms., and 

 was exposed at two openings each about a square centimetre 

 in area, and would thus be an " exposed " film. 



Probably the most striking feature in the comparison of 

 the results of the two methods is the nearness of the thickness 

 obtained in the second method to that in the first with an 

 exposed film. 



It may be remarked that the results of the second method 

 were all obtained before the second black films had been 

 noticed. Although it was impossible to see the second black 

 film in the second method, there can be no doubt but that the 

 first black broke immediately into the second; for such was 

 the case in the first method even when the exposure was ever 

 so much less. 



