58 THE PRINCIPLES OF SCIENCE. 



in a wedge-shaped glass, so that we have at a single 

 glance an infinite variety of thicknesses in view. As 

 Newton himself remarked, a red liquid viewed in this 

 manner is found to have a pale yellow colour at the 

 thinnest part, and it passes through orange into red, 

 which gradually becomes of a deeper and darker tint 1 . 

 The effect may be noticed even in a common conical wine- 

 glass. The prismatic analysis of light from such a wedge- 

 shaped vessel discloses the reason, by exhibiting the pro- 

 gressive absorption of different rays of the spectrum as 

 investigated by Dr. J. H. Gladstone 15 . 



A moving body may sometimes be made to mark out 

 its own course, like a shooting star which leaves a tail 

 behind it. Thus an inclined jet of water exhibits in the 

 clearest mariner the parabolic path of a projectile. In 

 Wheatstone's Kaleidophone the curves produced by the 

 combination of vibrations of different ratios are shown by 

 placing bright reflective buttons on the tops of wires of 

 various forms. The motions are performed so quickly 

 that the eye receives the impression of the path as a com- 

 plete whole, just as a burning stick whirled round pro- 

 duces a continuous circle. The laws of electric induction 

 are beautifully shown when iron filings are brought under 

 the influence of a magnet, and fall into curves correspond- 

 ing to what Faraday called the Lines of Magnetic Force. 

 When Faraday tried to define what he meant by his 

 lines of force, he was obliged to refer to the filings. *' By 

 magnetic curves/ he says 1 , ' I mean lines of magnetic 

 forces which would be depicted by iron filings.' Robison 

 had previously produced similar curves by the action of 

 frictional electricity m , and from a mathematical investiga- 



i 'Opticks,' 3rd edit. p. 159. 



1{ Watts, ' Dictionary of Chemistry/ vol. iii. p. 637. 



1 ' Faraday's Life,' by Bence Jones, vol. ii. p. 5. 



m Watts ' Dictionary of Chemistry,' vol. ii. pp. 402, 403. 



