EULOGY ON THOMAS YOUNG. 125 



If I call to mind how uiauy persons do not appreciate scientific theo- 

 ries except in proportion to the immediate applications which they may 

 offer, I cannot terminate this ennmeration of the phenomena which 

 characterize the several series of more or less nnmerous periodical col- 

 ors without mentioning the rings, so remarkable by their regularity of 

 form and purity of tint, with which every brilliant light appears sur- 

 rounded when we look at it through a mass of fine molecules or fila- 

 ments of equal dimensions. These rings, in fact, suggested to Young 

 the idea of an instrument, extremel.y simiile, which he called an "eri- 

 ometer," and with which we can measure, without difficulty, the dimen- 

 sions of the most minute bodies. The eriometer, as yet so little known 

 to observers, has an immense advantage over the microscope 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 magni- 

 tude among themselves, or, in other words, when the question of deter- 

 mining their dimensions has no real meaning. 



Young applied his eriometer to the measurement of the globules of 

 blood in different classes of animals ; to that of powders furnished by 

 different species of vegetables ; of the fineness of different kinds of fur 

 used in the manufacture of diiferent fabrics, from that of the beaver, 

 the most valuable of all, down to that of the common sheep of the Sus- 

 sex breed, which stands at the other extremity of the scale, and is com- 

 posed of filaments four times and a half thicker than that of the beaver. 



Before the researches of Young the numerous i^henomena of colors* 

 which I have just pointed out were not only inexplicable, but nothing 

 had been found to connect them with each other. ISTewton, who was 

 long engaged on the subject, had not perceived any connection between 

 the rings in thin films and the bands of diffraction. Y^uing reduced 

 these two kinds of colored bands alike to the law of interference. At 

 a later period, when the colored phenomena of polarization had been 

 discovered, he observed in certain measures of the thickness at which 

 they occurred some remarkable numerical analogies, which made it very 

 reasonable to expect that sooner or later this singular kind of polariza- 

 tion would be found connected with his doctrine. He had in this in- 

 stance, however, w^ must admit, a very wide hiatus to fill up. The 

 knowledge of some important properties of light, then comidetely un- 

 known, would have been necessary to permit him to conceive the whole 

 singularity of the effects which, in certain crystals cut in certain direc- 

 tions, double refraction produces by the destruction of light resulting 

 from the interference of rays ; but it is to Young that the honor belongs 

 of having opened the way; it was he who was the first to decipher 

 these hieroglyphics of optics.t 



* Every one may have remarked the threads of a spider's weh occasionally exhibiting 

 brilliant colors in the sunshine. The same thing is seen in fine scratches on the surface 

 of polished metal, produced in a more regular way by the fine engraved ijarallel 

 grooves in Barton's buttons. The colors of mother-of-pearl arc of the same kind ; all 

 these colors Dr. Young showed were due to interference of the portions of light reflected 

 from the sides of the narrow transparent thread or groove. — Translator. 



t It has been well observed that simplicity is not always a fruit of the first growth, 

 and accordingly some of the earliest of Young's researches were complicated by unne- 

 cessary conditions. Thus, to exhibit the effect of two rays interfering, he at first not 

 imnaturally transmitted the narrow beam of light through two suiall apertures near 

 together. In point of fact, though the real cfiect is here seen, it is mixed u]) Avith 

 others of a more complex kind. The narrow apertures each exhibited colored fringes 

 in addition to the interference stripes seen between them. The colored fringes of aper- 

 tures, unless very wide, are distinct from those formed by one external edge of an 

 opaque body, the light from each side conspires to the effects in a somewhat complex 



