METHOD OF VARIATIONS. 59 



tion of the forms of such curves we may infer that mag- 

 netic and electric attractions obey the general law of 

 emanation, that of the inverse square of the distance. In 

 the electric brush we have another similar exhibition of 

 the laws of electric attraction. 



There are several branches of science in which col- 

 lective experiments have been used with great ad- 

 vantage. Lichtenberg's electric figures, produced by 

 scattering electrified powder on an electrified resin cake, 

 so as to show the condition of the latter, suggested to 

 Chladni the notion of discovering the state of vibration of 

 plates by strewing sand upon them. The sand collects at 

 the points where the motion is least, and we gain at a 

 glance a comprehension of the general form of undulation 

 of the whole plate. To this method of experiment we owe 

 the beautiful observations of Savart. The exquisite 

 coloured figures exhibited by plates of crystal, when ex- 

 amined by polarized light, afford a more complicated 

 example of the same kind of investigation. They led 

 Brewster and Fresnel to a successful explanation of the 

 properties of the optic axes of crystals. The unequal 

 conduction of heat in crystalline substances has also been 

 shown in a similar manner, by spreading a thin layer of 

 wax over the plate of crystal, and applying heat to a 

 single point. The wax then melts in a circular or elliptic 

 area according as the rate of conduction is uniform or not. 

 Nor should we forget that Newton's rings were an early 

 and most important instance of investigations of the same 

 kind, showing the effects of interference of light undula- 

 tions of all magnitudes at a single view. Sir John 

 Herschel gave to all such opportunities of observing 

 directly the results of a general law, the name of Col- 

 lective Instances n , and I propose to adopt the name 

 Collective Experiments. 



n ' Preliminary Discourse,' &c., p. 185. 



