On Dr. Herschel's AO-feet Telescope. 339 



and retained at distances necessary for producing a colouring 

 dependent upon the thickness of these elementary parts. 



In the fourth and last place, the colours of pellicles are in 

 certain cases variable by the inclinations of light and of the 

 eye, and sometimes also by the influence of the mediums 

 with which we place them in contact. Nothing similar 

 takes place with respect to the colours peculiar to molecules : 

 fixed and permanent, in whatever direction we view them, 

 they do not even change upon being introduced into a new 

 limpid medium of a greater or less density. 



These, in my mind, are enough of characteristic differ- 

 ences to authorise the opinion that the colours of bodies m 

 masses have not the same origin with those of thin pellicles : 

 a conclusion equally important with respect to its object ni 

 itself, as in respect to the disputes it still occasions among 

 men of science*. 



[To be continued.] 



LIIL Jn Examination of what Jerome de Lalande has 

 published, in his History of Astronomy for 1806, con- 

 cerning Dr. Herschel and his 40-feet Telescope. 

 To Mr. Tilloch. 



TT ^^^' 



Having long taken in your valuable philosophical miscel- 

 lany, I lately met with the article called " The History of 

 Astronomy, by Jerome de Lalande, for 1806," which has 

 a place in your Magazine for July last. This is a new spe- 

 cies of composition, which, it is well known, that author 

 for several years past had been fond of: a kind of annual 

 bulletin, consisting of a great farrago of topics and titles 

 respecting the labours of his cotemporaries in astronomy 

 and its kindred sciences, all treated of in a very cursory man- 

 ner, with strictures on their merits not unfrequently super- 

 added. 



• Sec, among others, in the second edition of the Elements of Dyeing, by 

 M.BcrWiollet, and in Hauy's Treatise upon Physics (second edition), the 

 diKusMomand the contrary opinions of these celebrated authors upon this 

 question. _ 



Y 2 *^ 



