August 23, 1S83] 



NA TURE 



39i 



tions below the border of the yellow of the second order 

 (374X io" 6 mm. when the light is incident at 45 ). 



On the other hind, our electrical observations of a 

 black film often give the same thickness to within 1 or 

 2 per cent., again and again, in a series of observations 

 extending over an hour or more. 



This constancy may be taken as proving that the film 

 is not absorbing nor losing moisture, and if its compo- 

 sition thus remains unaltered it is not too much to say 

 that the electrical method extends to 7'2X icr 6 mm. (the 

 smallest thickness measured by us), with an accuracy 

 previously attainable only above 374X io -6 mm. In other 

 words, it carries the accurate measure of thickness fifty 

 times nearer molecular magnitudes than Newton's scale of 

 colours does. 



We now come to the interesting point raised by Sir 

 William Thomson, which wc may perhaps be allowed to 

 state in his own words as follows : — " Newton, in the pas- 

 sage I have quoted (Nature, vol. xxviii. p. 250), being 

 Observation 17 of the Second Book, Part I., of his 

 ' Optics,' says (1) he found in the large black spot smaller 

 black 'round' spots which were blacker still ; (2) he saw 

 sunlight reflected from even the small darker spots ; 

 (3) the black spots would break out in the middle of 

 white, without any intervention of blue, and sometimes 

 within the yellow or red or blue of second order. This 

 (3) agrees with your (1) of p. 151. But the (1) above of 

 Newton's shows a higher grade of thinness than that of 

 the main black spots, which I presume is that which you 

 have found as 1X10 s . I do not know if you have 

 noticed these smaller and blacker spots. It would be 

 exceedingly interesting if possible to find their thickness, 

 and to see how they seem to be related to the main black 

 spots." 



It may be well when answering this inquiry as to 

 whether we have observed the smaller black spots, to 

 state such facts as we have observed connected with the 

 formation of the black. 



In the first place we have noticed t'^at the boundary 

 between the film proper, and the small aggregation of 

 liquid which connects it with the solids by which it is 

 supported, is the place where, under ordinary circum- 

 stances, discontinuous spots, i.e. spots having a thickness 

 different from that of the surrounding film, are most readily 

 formed. The small circular masses of liquid which sur- 

 rounded the gold wires by which the film was connected 

 with the electrometer were sometimes themselves sur- 

 rounded by a very narrow ring, showing the white of the 

 first order when all the film immediately outside it was 

 much thicker. Small specks of white would frequently 

 break off from the topmost point of this ring, and either 

 rise through the film to its highest point, or if, as was 

 often the case, the liquid of the film was in a state of 

 internal motion, the white flecks would be carried 

 round the cylinder in spiral paths. Some liquids 

 almost invariably gave films which, shortly before rup- 

 ture, became thus covered with white flecks. Occasion- 

 ally a white band, several tenths of a millimetre in 

 breadth, was formed all round the upper ring which car- 

 ried the cylindrical film, when the portion of film next 

 it showed colours of the second and higher orders, and it 

 was owing (among other reasons) to the frequent presence 

 of this ring that we abandoned the Wheatstone's bridge 

 method used in our first experiments (Proc. Nov. Soc. 

 1877, p. 334), and adopted the electrometer method which 

 we now always employ. The necessity of having to make 

 an allowance for the resistance of the white ring, the thick- 

 ness of which was much more uncertain than that of the 

 coloured portion was thus avoided. We may remark that 

 irregularities of all kinds are more likely to occur if all 

 parts of the apparatus are not frequently and scrupulously 

 eleaned. We have also examined the lines of discon- 

 tinuity between the black and the coloured portions, 

 using a microscope with a three-inch object glass. In 



many cases the discontinuity was seen to be only 

 apparent. Bands of colour were visible, which proved 

 that the missing tints were really there, but on so small a 

 scale as to be invisible to the naked eye. 



The phenomenon of the white band was sometimes 

 still further complicated by the presence of spots in the 

 white, differing in colour both from it and the film next it. 

 Thus on one occasion when the colour next the white 

 was the green of the fourth order (mean thickness 

 893 Xio -6 mm.) we made the following remarks in our 

 notebook : — " At the top a narrow film of white was 

 observed between the green and the solid cylinder. In 

 this, small pieces of deep blue were moving slowly back- 

 wards and forwards. The lower part of the white was 

 marked by two small rings of colour, so narrow that the 

 colours were indistinguishable." Later on, the green at 



^ B 



Fig. 1. 



one point broke through the white and completed contact 

 with the top. This contact was in turn broken, and, after 

 a while, the white appeared continuous without spots or 

 bridges. A rough, highly enlarged sketch of the spots 

 was made at the time, of which Fig. t is a copy. 



a B is the lower edge of the platinum cup which sup- 

 ports the film. A c and B D are the boundaries of the 

 bright line produced by the light thrown upon the film at 

 a known angle for the purpose of measuring its thickness. 

 E F is the edge of the white. Two of the blue spots 

 which appeared to float in it are shown, and the narrow 

 line of colour is indicated. Below e f the thickness was 

 about nine times greater than that in the white space 

 above it. 



Our reason for describing this observation at length is 

 to draw attention to the curious phenomenon of the blue 

 spots separated by an apparent discontinuity from both 



the white and the green. From the green they were no 

 doubt separated by a thin line of white, and through this 

 frail band, perhaps a few hundredths of a millimetre 

 broad and one ten-thousandth of a millimetre thick, they 

 were unable to sink into the green below. 



In the same way white flecks have been observed to 

 rise and to be separated for some seconds from the white 

 ring above by a thin band of colour. Such flecks, exa- 

 mined by the microscope, sometimes show colours of 

 higher orders within them, arranged in curves owing to a 

 regular vortical circulation. The appended sketch (Fig. 2), 

 drawn from memory, gives some idea of the appearance 

 displayed, the size being of course greatly exaggerated. 



Turning now from the formation of the white to that 

 of the black, many of the phenomena observed in the 



