Memoir of Dr. Thomas Young. 325 



over in physical treatises ; and lastly, those crowns with 

 constantly changing diameters which often appear to sur- 

 round the sun and moon. 



When I recal to mind the limited number of persons who 

 can appreciate scientific theories unless they immediately 

 apply them, I could not terminate this enumeration of 

 phenomena which characterize the more or less numerous 

 series of periodical colours, without mentioning the rings 

 so remarkable for the regularity of their form, and by the 

 purity of their lustre with which all bright light appears 

 surrounded, when it is examined through a mass of mole- 

 cules or filaments of equal dimensions. These rings, in fact, 

 suggested to Young the idea of an extremely simple instru- 

 ment which he called eriometer with which the measurement 

 of the smallest bodies can be readily effected. The erio- 

 meter still so little known to observers has this immense 

 advantage over the microscope, that it gives at once the 

 mean size of millions of particles comprized within the 

 field of vision. It possesses besides the singular property 

 of becoming negative when the particles differ too much 

 from each other, or, in other words, when the question of 

 determining their dimensions cannot be answered. 



Young applied his eriometer to measure the globules of 

 the blood of different classes of animals, those of the dust 

 furnished by different species of vegetables, to the mea- 

 surement of the fur employed in the manufacture of 

 stuffs, from that of castor the finest of all, to that of the 

 fleeces of the common flocks of the county of Sussex, 

 which, placed at the extremity of the scale, are composed 

 of filaments four and a half times larger than the hair of 

 the castor. 



Before^ Young, the numerous phenomena of colour which 

 have just been pointed out were not only unexplained, but 

 there was nothing written on the subject. Newton, who 

 had long been occupied with the subject, had not, for ex- 

 ample, perceived any connexion between the iridescence of 

 their plates and the bands of diffraction. Young found 

 these two species of coloured striae to be only the effects of 

 interference. Subsequently, when chromatic polarization 

 was discovered, he brought up in some measurements of 

 thickness some remarkable numerical analogies, well fitted 



