I 1(5 USEFUL NOTICES. 



etching should be omitted, and the writing made upon soft 

 metal with a sharp point leaving the bur on. Such a plate 

 would afford many impressions. 



It would be a great improvement upon Watt's method, if 

 the Counter-proofs could be taken upon dry paper. The 

 tracing paper of Wedgwood and the engravers soon loses 

 its colour, and it will not keep long. It soon becomes too dry 

 to give off its colour. 



2. Scintillation of the Stars. 



Twinkling of Many speculations have been offered to account for and explain 

 the stars as- r ..... /- ,- i r 



cribed to the that apparently irregular and agitated emission of light, from 



» ir « the fixed Stars, which has been called scintillation or twinkling. 



From its marked appearance at low attitudes, and almost 

 total absence at higher, it has been commonly ascribed to the 

 interposed atmosphere j which, by the changeable densities 

 of its parts, and the interposition of opake particles, is imagined 

 to produce variations in the quantities, colours, and directions 

 of the light before it arrives at the eye. In^proof of this doc- 

 trine it has been farther noted, that the stars do not scintillate in 

 a telescope. Undoubtedly the effect is still clouded with un- 

 certainty. An observation I made upon the Dog Star (Sirius) 

 in the autumn of ISO/ may be considered as affording a few 

 facts more m addition to those we already possess. 



The stars do It is not true that the stars have no scintillation in a tele- 



Stelwope* m scope. It maybe strikingly observed by putting the instru- 

 ment out of adjustment. In this case the circular disc of 

 light, has a kind of vaccillation, as if a number of discs were 

 continually flashing before each other : the illumination seemed 

 to come on at different sides, and these discs also differ in colour. 



but give co- Blue, steel blue, pea-green, bright copper, red and white, are 



loured rays in amone the most usual colours: but the rapidity of succession 

 succession. » ' , , , i 



does not allow the sense to determine whether these colours 



may be more or less cotemporaneous, or completely and dis- 

 tinctly succeeding each other. To determine this point, I 

 An experi- took an achromatic glass of Ramsden's, magnifying 24 times, 



coloured ^9 and direcled lt to tbe star — l ^ e ob J ect en(1 bein S supported 

 of the Dog in a notch in a steady bar connected with the wtfU^&nd the eye 

 Slar * end, upon an adjustable piece wnich was likewise capable of 



S ..being 



