THE EXACT MEASUREMENT OF PHENOMENA. 335 



length and thinness of the supporting thread. Forces so 

 minute as the attraction of gravitation between two balls, 

 or the magnetic and diamagnetic attraction of common 

 liquids and gases, may thus be made apparent, and even 

 measured. The common chemical balance, too, is capable 

 theoretically of indefinite sensibility 8 . 



The third mode of measurement, which may be called 

 the Method of Eepetition, is of such great importance and 

 interest that we must consider it in a separate section. It 

 consists in multiplying both magnitudes to be compared 

 until some multiple of the first is found to coincide very 

 nearly with some multiple of the second. If the multipli- 

 cation can be effected to an indefinite extent, without the 

 introduction of countervailing errors, the accuracy with 

 which the required ratio can be determined is unlimited, 

 and we thus account for the extraordinary precision with 

 which intervals of time in astronomy are compared to- 

 gether. 



The fourth mode of measurement in which we equate 

 submultiples of two magnitudes is comparatively seldom 

 employed, because it does not conduce to accuracy. In 

 the photometer, perhaps, we may be said to use it ; we 

 compare the intensity of two sources of light, by placing 

 them both at such distances from a given surface, that the 

 light falling on the surface is tolerable to the eye, and 

 equally intense from each source. Since the intensity of 

 rays diminishes, as the inverse squares of the distances, 

 the relative intensities of the luminous bodies are propor- 

 tional to the squares of their distances. The equality of 

 intensity of two rays of similarly coloured light, may be 

 most accurately ascertained in the mode suggested by 

 Arago, namely, by causing the rays to pass in opposite 

 directions through two nearly flat lenses pressed together. 



8 Watt's 'Dictionary of Chemistry/ art. Balance, vol. i. p. 487. 



