428 PHILOSOPHICAL TRANSACTIONS. [aNNO 1773, 



likewise ascertained, to reduce the instant of the observed eclipse to that of the 

 passage of the centre over the edge of the shadow, which will be a fixed term for 

 all the observations, and all the observers, who but seldom agree in the obser- 

 vation of the same eclipse. I confess that the transparency of the air is not al- 

 ways the same, and that a greater or less degree of transparency will make the 

 segments smaller or larger, and consequently affect the observation. The in- 

 equality of sight may likewise occasion some error ; for though it might be pos- 

 sible to settle the general effect of the difference of sight of different observers, 

 the sight of the same person is not constantly the same, and even independently 

 of the ^change produced by age, may not have the same strength at all times. 

 But by th4 method I, propose, all these inconveniencies will be remedied, in 

 future observations, with little trouble. Every observer is to furnish himself 

 with several diaphragms of pasteboard, gradually diminishing by half-lines, to be 

 applied to the object-glass externally, and some minutes before an immersion, or 

 after an emersion, he' is to determine which of them intercepts from him the 

 sight of the satellite. Having found this, and knowing also the diameter of the 

 satellite, he will reduce, by the process of calculation already explained, the ob- 

 .served instant of the eclipse, to that of the passage of the centre; which is the 

 same, as I said before, for all the observers in the world. You see, sir, what 

 advantages would arise, from this agreement, for the theory of the satellites, and 

 the precision of the terrestrial longitudes. This method takes in every thing; 

 the difference of glasses, that of sights, the greater or less transparency of the 

 atmosphere, &c. Observation gives the segment greater or less, in proportion 

 to the combined influence of all these causes. The principal advantage of this 

 method, which requires only a very simple calculation, is, that it depends on no 

 hypothesis. It enables us to measure immediately the light of the satellite, whe- 

 ther increased or diminished by all the causes above mentioned ; to measure, I 

 say, the real impression of that light upon the eye, whatever be the actual state 

 of the organ. I must add, that I am sensible, the determination of the invisible 

 segment, by means of the diaphragm, might be inconvenient to those, who make 

 use of large telescopes for the eclipses of the satellites, were it not, that this ob- 

 servation may be equally well made with a smaller telescope, provided only, that 

 it be sufficient to see and distinguish the 4 satellites; and after the diaphragm is 

 determined by this smaller telescope, the larger one may be used for the observa- 

 tion of the eclipse. For these measures are easily transferred from one instru- 

 ment to another, the invisible segments in different telescopes, being inversely as 

 the squares of the apertures. For reflectors, I have a method of the same kind 

 with the former, grounded at least on the same principles, by which I can de- 

 termine their power, and compare them, both with each other, and with the 

 refracting telescopes. I shall conclude with some hints concerning the observa- 



