462 PHILOSOPHICAL TRANSACTIONS. ' [aNNO J 754. 



of the first satellite of Jupiter, and which times we have seldom found to differ 

 from the observations above lO': by comparing therefore the time of the emer- 

 sion of the first satellite observed by this gentleman, with the time computed 

 from these formulae, the difference of longitude between London, at St. Paul's, 

 and the place of observation at Lisbon, comes out to be 36"" 6^ ; and by several 

 former corresponding observations the difference had been found to be 36™ lO'. 



M. Chevalier further mentions the observation of the eclipse of the sun last 

 October, through a telescope of 1 5 palms. He saw both the beginning and 

 end, in a very clear air ; and says that the greatest quantity of the eclipse was 1 1 

 digits and 5', which he measured with a micrometer ; but unluckily he has not 

 given us either the diameter of the sun, or that of the moon, which he might 

 have measured, (for the eclipse was annular) though he was at the pains of mea- 

 suring all the digits, both in the increase and decrease of the eclipse. He further 

 takes notice, that at the time of the greatest obscuration, the light of the sun 

 was remarkably diminished ; and that they were able to see Jupiter, Venus, and 

 some stars of the first and 2d magnitude ; but he could not see Mercury, on 

 account of his proximity to the sun : and that a reflecting speculum, of 3 palms 

 in diameter, which could melt lead, when placed in its focus, and instantly set 

 wood in a flame, produced the same effects, even when the sun was 7 digits 

 eclipsed ; but that, about the time of the greatest obscuration it was not able to 

 burn wood, though held in its focus for some time : and that at the same time 

 the air became very cold, the wind blowing hard from the north ; and that some 

 vapours, or fog, were seen to rise out of the river and adjacent harbour. 



LXXIf^. Of an Instrument for Measuring Small Angles, the first Account of 

 which was read before the Royal Society, May 10, 1733. By Mr. John 

 Dollond. Dated April A, 1754. p. 551. 



Before entering on particulars relating to this micrometer, it will be proper to 

 make a few preparatory observations on the nature of spherical glasses, so far as 

 may be necessary to render the following explanation more easily understood. 



Obs. 1 . — It is a property of all convex spherical glasses, to refract the rays of 

 light which are transmitted through them, in such a manner, as to collect all 

 those that proceed diverging from any one point of a luminous object, to some 

 other point ; whose distance from the glass depends chiefly on its convexity, and 

 the distance of the object from it. 



Obs. 2. — The point where the rays are thus collected, may be considered as 

 the image of that point from which they diverge. For if we conceive several 

 radiant points thus emitting rays, which, by the refractive quality of the glass, 

 are made to converge to as many other points ; it will be an easy matter to under- 

 stand how every part of the object will be truly represented. As this property of 



