—=—S = ee 
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 sz/ent 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.—Philos. Mag. May, 1855. 
