146 Remarks on Dr. Enfield's Institutes 



possess an excess of refraction, without also possessing an 

 excess of dispersive power ? For the removal of this diffi- 

 cultj^io hint is given of the great fact which lies at the 

 foundation of Dollond's improvements, viz. that the dis- 

 persive power of different media is not proportioned to 

 their mean refractive power. Unless he has the sagacity 

 to conjecture that this may be the case from the obscure 

 statement above quoted, he will remain in ignorance of what 

 has been justly regarded as the greatest discovery made in 

 Optics since the period of Newton. 



Book VII. We had hoped to see the numerical express- 

 ions for the principal magnitudes, distances, and angles 

 which occur in Astronomy, corrected in the present edition 

 so as to correspond with the present state of the science. But 

 with the exception of two or three manifest errors of the 

 press, and the general table at the end of part I., the num- 

 bers of former editions, many of which are a century old, 

 are retained. We will therefore, once for all, give the cor- 

 rections of those which may be considered as ascertained 

 with the most precision. The student can, if he choosesj 

 go over them with his pen. 



^ These numbers are Fianisteed's. The corrections are given from Lap- 

 lace. 



