NOTES TO BOOK IX. ."307 



by the operation of acids, is not gum, and has a very 

 energetic right-handed effect. This substance M. Biot 

 called dextrine, and he has since traced its effects into 

 many highly curious and important results. 



(KA.) p. 444. I may mention, in addition to the 

 applications which Young made of the principle of inter- 

 ferences, his Erlometer, an instrument invented for the 

 purpose of measuring the thickness of the fibres of wood ; 

 and the explanation of the supernumerary bands of the 

 rainbow. These explanations involve calculations founded 

 on the length of an undulation of light, and were confirmed 

 by experiment, as far as experiment went. 



(LA.) p. 458. In the Phil. Ind. Sc. B. xi. Ch. iii. 

 Sect. 11. I have spoken of the consilience of inductions as 

 one of the characters of scientific truth. We have several 

 striking instances of such consilience in the history of the 

 undulatory theory. The phenomena of fringes of shadows 

 and coloured bands in crystals jump together in the Theory 

 of Vibrations. The phenomena of polarization nd double 

 refraction jump together in the Theory of Crystalline Vi- 

 brations. The phenomena of polarization and of the inter- 

 ference of polarized rays jump together in the Theory of 

 Transverse Vibrations. 



(MA.) p. 466. The proof of what is here said of the 

 undulatory theory is contained in the previous history. 

 This theory has " accounted for, explained, and simplified 

 the most entangled cases ;" as the cases of fringes of 

 shadows ; shadows of gratings ; coloured bands in biaxal 

 crystals, and in quartz. There are no optical phenomena 

 more entangled than these. It has "corrected experi- 

 mental laws," as in the case of M. Biot's law of the 

 direction of polarization in biaxal crystals. He has done 



