480 LECTURE XL. 



phori, and the refraction of the rarest of natural bodies would be incompa- 

 rably greater than that of the densest is actually found to be : and on the 

 whole, although the character of Euler has been so highly and so deservedly 

 respected as to attach a certain degree of authorit}' to all his opinions, so that 

 in this instance the name of Iluygens has been almost superseded by that of 

 Euler, yet in fact he has added no argumentative evidence whatever to the 

 theory, but, by inaccurate and injudicious reasoning, has done a real injury 

 to the cause which he endeavoured to support. 



The researches of Lambert may be considered as a continuation of those of 

 Bouguer; they present us with many interesting observations on the natural 

 history of light, and the properties of various bodies with regard to it. Mr. 

 Lambert first ascertained that a luminous surface emits its light very nearly 

 with eqvial intensity in all directions, so that any part of it appears almost 

 equally brilliant to an eye placed in any direction, while the light thrown 

 by each square inch or square foot of the surface in any direction differs ac- 

 cording to the obliquity of that direction. The mathematical theory of 

 optics is considerably indebted to the labours of Clairaut, Dalembert, and 

 Boscovich ; Jeaurat, Beguelin, Redern, and Kliigel have also continued the 

 investigation; their calculations may be of considerable utility to the prac- 

 tical optician, but it requires the ingenuity of a Dollond or a Ilamsden to 

 apply the whole of the results to any useful purposes. 



The experiments of Maz6as on the colours of thin plates are mere repeti- 

 tions of those of Newton under disadvantageous circumstances; Mr. Dutour 

 has, however, considerably diversified and extended these experiments, as 

 well as those on the colours which are produced in diffracted light, yet without 

 obtaining any general results of importance. Comparetti's experiments on 

 inflection have every appearance of accuracy, but they are much to^o in- 

 tricate to be easily compared with each other, or with those of former 

 observers. 



The late Dr. Priestley rendered an essential service to'.the science of optics, 

 considered as a subject for the amusement of the general reader, by an 

 elegant and well written account of the principal experiments and theories, 

 which had been published before the year 1770. But this work is very defi- 



