OPTICS. 



33 



CHAPTER XIII. Colours of Thin 

 Plates Solids Fluids A ir A>.v- 

 ton's Table of the Colours of Thin 

 Plates Theories of the Phenomena. 

 EVERY person must have observed that 

 the light reflected from, and transmitted 

 through, transparent and colourless 

 bodies, such as flint glass, and water, 

 &c., is always white, provided that in 

 the case of the transmitted light the 

 two surfaces of the body are parallel. 

 This is true for all the different thick- 

 nesses of these bodies which we are in 

 the habit of observing, but if we di- 

 minish their thickness more and more, 

 we shall at last arrive at a thickness 

 where both the reflected and the trans- 

 mitted light becomes coloured. 



In solid bodies, such as glass, this is 

 not easily accomplished, but in mica, a 

 thin platy mineral, it is easily effected. 

 If we stick one side of a piece of mica 

 to sealing wax, and again tear it away 

 with a jerk, we shall find some very 

 thin films left on the wax, some of 

 which will reflect a brilliant red, others 

 a brilliant yellow, and others a bright 

 blue. AYe may accomplish the same 

 object, perhaps better, by taking the 

 thinnest film that can be split from 

 gypsum, or sulphate of lime, and im- 

 mersing it in a vessel of water. The 

 water will dissolve the sulphate of lime 

 most at the edge, so that we shall have 

 the film shading off in thickness to the 

 finest edge. At this edge will be seen 

 fringes of colour corresponding to the 

 different thicknesses of the film. If the 

 film could be made thin enough, we 

 should arrive at a point when it would 

 cease to reflect any light, and when the 

 whole light which fell upon it would be 

 transmitted. This has never been done 

 artificially in solid bodies, and probably 

 never will be. 



Accident, however, on one occasion 

 accomplished what was beyond the 

 reach of skill, and exhibited, perhaps, 

 the most curious optical fact that has 

 ever been witnessed. A crystal of 

 quartz, about 2| inches in diameter, 

 having been broken in two, the faces of 

 the fracture appeared absolutely black, 

 like black velvet. This was ascribed 

 by those who saw it, to a thin film of 

 minutely divided opaque matter which 

 had insinuated itself at a crack in the 

 stone. Upon examining it, however, by 

 various optical methods, Dr. Brewster 

 found that the blackness was owing to 

 a. fine down of quartz, the diameter of 

 the fibres of which was so minute, that 



they were incapable of reflecting light. 

 The diameter of these delicate fibres, as 

 will be afterwards seen, could not ex- 

 ceed the one-third of the one-millionth 

 part of an inch* This remarkable 

 specimen belongs to the cabinet of her 

 Grace the Duchess of Gordon. 



In fluid bodies it is much more easy 

 to observe the colours produced when 

 their thickness is greatly reduced. If 

 we blow a soap bubble, and cover it 

 with a clear glass, we shall observe a 

 great many concentric coloured rings 

 round the top of it. As the bubble 

 grows thinner, the rings grow wider, and 

 at last, before the bubble bursts, there 

 will be seen at the top of it a small 

 round black spot, which will expand 

 itself to i or |ths of an inch. The same 

 phenome'na may be seen at the mouths 

 of bottles containing oil of turpentine, 

 alcohol, and many of the essential 

 oils, where it is easy to form a film of 

 the fluid, in which 'the colours will be 

 seen to great advantage. The experi- 

 ment may be still more easily made by 

 putting a thin film of any evaporable 

 fluid upon a clean plate of glass, and 

 observing the colours at the edges of the 

 film, and just before it is dried up. 



The colours of thin films have been 

 chiefly studied when formed by thin 

 plates of air. In order to exhibit them 

 distinctly, two convex lenses, A B, CD, 

 of long focal lengths, are placed the one 

 above the other, so as to touch at their 

 summits. Three pair of screws, p. p,p,a.i-& 

 -> used, to keep the 



Fig. 35. fjz . lenses together, 

 and to produce a 

 regular pressure at 

 the point where the 

 lenses touch each 

 other. Sir Isaac 

 Newton used a 

 plano-convex lens, the radius of whose 

 convex surface was 23 feet, and a double 

 convex one, the radius of each of whose 

 surfaces was 50 feet. The first was 

 placed, like C D,with its plane side down- 

 wards, and the other, A B, was placed 

 above it. By pressing the lenses together 

 there appeared round the point of 

 contact a regular system of circular 

 coloured rings or spectra, having a black 

 spot in the centre, each spectrum, or 

 order of colours, consisting of fewer 

 colours as they receded from the centre. 

 Upon examining: the light transmit- 

 ted through the lenses, Sir Isaac ob- 



See the Edinburgh Journal of Science, No. I, p. US 



D 



