350 Miss H. Gr. Klaassen on Change of Phase on Reflexion 



thickness, which exhibit not only this colour but a series of 

 different colours. 



The best method I have yet found of making such films is 

 to pour a little alcoholic solution of required concentration 

 (found by trial) upon a piece of glass, then to wave the glass 

 through the flame of a Bunsen burner. The fuchsine does 

 not melt : but the alcohol is rapidly removed and leaves an 

 amorphous film of the dye. If the concentration and quantity 

 are rightly chosen, the film has the appearance of a brilliant 

 mirror of variegated colour. The following table gives the 

 colour of the film as seen by reflected light at normal in- 

 cidence. The colours are given in order of increasing thick- 

 ness of film. The Roman numerals are used for reference 

 only ; they are not a measure of the thickness of the film : — 



Table A. 



Thickness 

 of film. 



I. 



Colour of film as 

 seen directly. 



Like glass. 

 Greenish. 



Straw-colour. 

 Blue. 



Yellowish green. 

 Green. 



II. 



Colour of the film as seen from the back 

 of the glass on which it is deposited. 



I. 

 II. 



III. 



IV. 



V. 



VI. . . 



Like glass. 



Bluish neutral colour, gradually changing 



with increasing thickness to deep red. 

 Bright orange. 

 Blue. 

 Pink. 

 Bluish green. 



§ 2. Examination of Film in Monochromatic Light reflected 



at Normal Incidence. 

 Red Light. — Fuchsine is most transparent to red light. 

 Numbers of interference-bands can be seen in this colour. 

 In Table A: 



III. is a thickness which gives maximum brightness in red 

 light ; 



IV. is a thickness which gives minimum brightness in red 



light ; 



V. is a thickness which gives maximum brightness in red 



light- 



The maxima and minima appear to be in the same positions 

 if we look at the film directly or if we see it through the 

 glass on which it is deposited. 



Yellow Light. — Only a few maxima and minima can be 

 distinctly seen. If we take an extremely thin film in which 

 the gradient of thickness is very small, looking directly at the 

 film, the intensity rises at first with increase of thickness, 

 reaching a maximum in thickness III. near the corresponding 



