Mechanical Science. 165 



apparent coloured bands. The light may be held a few inches 

 before the eye, and so that the incident and reflected rays may 

 make but a small angle. This experiment is due to Mr. Whewell 

 of Cambridge ; but M. Quetelet, on repeating it, found that it was 

 not constantly produced, and that the necessary condition was the 

 presence of a slight film of vapour on the glass*. To make the 

 experiment it is sufficient to breathe upon a cold mirror at the place 

 where the image of the candle is to be reflected. 



M. Quetelet has found that the experiment succeeds as well 

 when the mirror is not silvered ; even a piece of crown glass will 

 do ; but, from its irregularities, the bands are not so distinct. Day- 

 light does not interfere with the observation. A drop of oil behind 

 the glass makes the colours disappear. A line from the image of 

 the eye to the image of the light is always perpendicular to the 

 direction of these coloured lines. The bands affect the form of 

 curved lines, which, in certain cases, degenerate into straight linesf. 

 They do not extend far beyond the image of the light. The colours 

 proceeding from the light are bluish-green, yellow, red ; bluish- 

 green, yellow, red, &c. Other circumstances being the same, the 

 bands are larger as the observer is farther from the mirror, as the 

 ligfit is nearer to the eye, and in fact as the angle between the inci- 

 dent and the reflected rays is smaller. 



This phenomenon does not appear to be related to that which 

 Newton observed with concave mirrors. It appears, as to the 

 colours, to have more connexion with the effect observed when the 

 sun or a light is seen through a transparent plate upon which has 

 been spread a very fine powder. 



The breath forms but a transient haze ; but M. Quetelet has 

 found an easy mode of rendering the preparatory state of the glass 

 permanent. It consists in extending a very thin regular film of 

 fatty matter, as oil or tallow, over the glass ; a soft cloth is to be 

 pressed lightly or dabbed over the whole surface of the film to 

 destroy the parallel lines otherwise existing, and then the effect is 

 obtained as well as with the breathj. 



9. SIZE FOR ILLUMINATORS, ARTISTS, &c. 



.Four ounces of Flanders glue and four ounces of white soap are to 

 be dissolved on the fire in a pint of water, two ounces of powdered 

 alum added, the whole stirred and left to cool. It is to be spread 

 cold with a sponge or pencil on the paper to be prepared, and is 

 much used by those who have to colour unsized paper, as artists, 

 topographers, &c. 



* Many mirrors produce the effect without the film, in consequence of a 

 slight granulation left upon the surface of the glass by the manufacturer. Ed. 



f We have never seen the bands in a flat piece of glass otherwise but straight. 

 -Ed, 



I Bull. Univ., A. xiii., 190, 192. Ibid. E. xiv. 344. 



