ON THE HISTORY OF OPTICS. 381 



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 equal 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 according to the obliquity of that direction. The mathe- 

 matical theory of optics is considerably indebted to the labours of 

 Clairaut,* Dalembert,t and Boscovich ; ^ Jeaurat, Beguelin,|| Redern,*[ 

 anc 1 , Kliigel** have also continued the investigation ; their calculations 

 may be of considerable utility to the practical optician, but it requires the 

 ingenuity of a Dollond or a Ramsden to apply the whole of the results to 

 any useful purposes. 



The experiments of Mazeas ft on the colours of thin plates are mere re- 

 petitions of those of Newton under disadvantageous circumstances ; Mr. 

 DutourJ^ has, however, considerably diversified and extended these expe- 

 riments, as well as those on the colours which are produced in diffracted 

 light, yet without obtaining any general results of importance. Compa- 

 retti's experiments on inflection have every appearance of accuracy, but 

 they are much too intricate 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 deficient in mathematical accuracy, and the author was not suffi- 

 ciently master of the science to distinguish the good from the indif- 

 ferent. 



Mr. Delaval's^T^T experiments on colours appear to show very satisfac- 

 torily, that all the colouring substances, in common use, owe their tints to 

 rays, which are separated from white light, during its passage through 

 them, and not, as Newton supposed, to the reflection of a particular colour 

 from the first surface. It has been observed that Kepler and Zucchius *** 

 had long ago made experiments nearly similar to those of Mr. Delaval. 



* Hist, et Mem. de Paris, 1756, vii.380, H. 112; 1757, p. 524, H. 153; 1762, 

 p. 578, H. 160. 



t Ibid. 1764, p. 75, H. 175; 1765, p. 53, H. 119; 1767, p. 43, H. 153. 

 Opuscules, vol. i. Hist, et Mem. de Berlin, 1769, p. 254. 



t Com. Bon. v. II. 265. 



Hist, et Mem. de Paris, 1770, p. 461, H. 103. 



|| On the Improvement of Telescopes, Hist, et Mem. de Berlin, 1762, pp. 66, 

 343 ; 1764, p. 7 ; 1769, p. 1 ; 1784, H. 40. 



H On Object-Glasses, ibid. 1759, p. 89 ; 1760, p. 3; 1761, p. 3. 



** On do. Comm. Gott. 1795, xiii. 2, 28. Gilbert's Annalen, xxiv. 265, 276. 

 Analytische Dioptrik, 4to, Leipz. 1778. 



ft Hist, et M6m. de Berl. 1752, p. 262. Mem. des Savans Etrangers, ii.26. 



U Ibid. vols. iv. and v. Rozier's Journal, i. 368; ii. 11, 249; v. 120, 230; 

 vi. 135, 330, 341,412. 



De Luce Inflexa et Coloribus, 4to, Pad. 1787. 



HI) The History and present State, &c. of Vision, Light, and Colours, 4to, Lond. 

 1772. 



HH Manch. Mem. ii. 131. *** Optica Philosophia, 2 vols. 4to, Lugd. 1652-6. 



