Additional Notes. 465 



Sounds in dijjcrtnt Gasts. p. 50. 



That the different gases have different degrees of power in 

 tlie propagation of sound, both with respect to intensity and 

 tone, has been known slnie the year HSG, about which 

 time Dr. Priestley, and Professor Perolle, of Turin, 

 instituted a set of experiments on this subject, in which they 

 substantially agreed. Since that time Professor J acqu in, of 

 Vienna, at the desire of Dr. Chladni, undertook a new 

 course of experiments, with a view to the investigation of 

 this subject. The results of" these experiments are so dif- 

 ferent, and even contradictory, when compared with the 

 former, that it is difficult to say on which side the truth lies. 



Optics. 

 Achromatic Telescope, p. 54. 



it appears that Dolland was not the first person who 

 invented Achromatic glasses. As early as 1729, Chester 

 More Hall, Esq. of More-Hall, in the county of Essex, 

 in South-Britain, as appears by his papers, considering the 

 different humours of the eye, imagined they were placed so 

 as to correct the different refrangibility of light. He then 

 conceived, that if he could iind substances having such pro- 

 perties as he supposed these humours might possess, he should 

 be enabled to construct an object glass that would show ob- 

 jects colourless. After many experiments, he had the good 

 fortune to find these properties in two different kinds of glass; 

 and by forming lenses made of such glass, and making them 

 disperse the rays of light in contrary directions, he succeeded. 

 About 1733 he completed several achromatic object glasses, 

 (though he did not give iliem tliis name) that bore an aper- 

 ture of more than two and an half inches, thougii the focal 

 length did not exceed twenty inches. One of these glasses, 

 which, in 1790, was in possession of the Rev. Mr. Smith, 

 of Charlotte-street, Rathbone Place, London, has been ex- 

 amined by several gentlemen of eminence in the scientific 

 world, and found to possess the properties ot the present 

 Achromatic glasses. 



In the trial at Westminster-Hall, about the patent for mak- 



ing Achromatic Telescopes, Mr. Hall was allowed to be 



