ERIOMETER. 311 



that of powders furnished by different species of vegeta- 

 bles, of the fineness of different kinds of fur used in the 

 manufacture of different fabrics, from tliat of tlie beaver, 

 the most valuable of all, down to that of the common 

 sheep of the Sussex breed, which stands at the other 

 extremity of the scale, and is composed of filaments four 

 times and a half thicker than that of the beaver. 



Before the researches of Young, the numerous phe- 

 nomena of colours* Avhich I have just pointed out were 

 not only inexplicable, but nothing had been found to 

 connect them with each other. Newton, who was long 

 engaged on the subject, had not perceived any connection 

 between the rings in thin films and the bands of diffrac- 

 tion. Young reduced these two kinds of coloured bands 

 alike to the law of interference. At a later period, when 

 the coloured phenomena of polarization had been discov- 

 ered, he observed, in certain measures of the thicknesses 

 at which they occurred, some remarkable numerical analo- 

 gies, Avhich made it very reasonable to expect that sooner 

 or later this singular kind of polarization would be found 

 connected with his doctrine. He had in this instance, 

 however, we must admit, a very wide hiatus to fill up. 

 The knowledge of some important properties of light, 

 then completely unknown, would have been necessary to 

 permit him to conceive the whole singularity of the effects 

 which in certain crystals, cut in certain directions, double 



* Every one may have remarked the threads of a spider's web occa- 

 sionally exhibiting brilliant colours 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 parallel grooves in Barton's 

 buttons. The colours of mother-of-pearl are of the same kind; all 

 these colours Dr. Young showed were due to interference of the por- 

 tions of light reflected from the sides of the narrow transparent thread 

 or groove. — Translator. 



