310 THOMAS YOUNG. 



number immediately under the innermost of the prismatic 

 bands of the rainbow,* and which seemed so completely 

 inexplicable, that the writers of elementary books on 

 physics had given up making mention of them ; and lastly, 

 the "coronas," or broad coloured circles with varying 

 diameters, which often appear surrounding the sun and 

 moon. 



If I call to mind how many persons do not appreciate 

 scientific theories, except in proportion to the immediate 

 applications which they may offer, I cannot terminate 

 this enumeration of the phenomena which characterize 

 the several series of more or less numerous periodical 

 colours, without mentioning the rings, so remarkable by 

 their regularity of form and purity of tint with which 

 every brilliant light appears surrounded, when we look 

 at it through a mass of fine molecules or filaments of 

 equal dimensions. These rings, in fact, suggested to 

 Young the idea of an instrument, extremely simple, 

 which he called an " eriometer," and with which we 

 can measure without difficulty the dimensions of the most 

 minute bodies. The eriometer, as yet so little known to 

 observers, has an immense advantage over the micro- 

 scope in giving at a single glance the mean magnitude of 

 millions of particles which are contained in the field of 

 view. It possesses, moreover, the singular property of 

 remaining silent when the particles differ much in mag- 

 nitude among themselves, or, in other words, when the 

 question of determining their dimensions has no real 

 meaning. 



Young applied his eriometer to the measurement of 

 the globules of blood in different classes of animals, to 



* This explanation has been recently controverted by Professor 

 Potter. Phibs. Mag. May, 1855. 



