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XIX. On the parallax of the fixed stars. By J. F. W. Herschel, 
Esq. M. A. Sec. R. S. Communicated January 19, 1826. 
Read March 9 and 16, 1826. 
The determination of the existence and amount of annual 
parallax in the fixed stars, was the object which originally 
drew the attention of astronomers to the examination and 
measurement of double stars, upon a principle suggested 
originally by Galileo, and improved on and more fully 
developed by my Father in one of his earliest communications 
to the Royal Society, Phil. Trans. 1782 ; according to which, 
two stars placed very nearly in a line with each other and 
with our system, ought, if situated at very different distances, 
in the line of sight, to be subject to periodical variations in 
their apparent distance from each other, according as the 
earth in its annual motion approaches to or recedes from that 
point in the plane of the ecliptic, where the line joining them, 
prolonged, would meet it. The difficulty of determining the 
distance of two stars of a double star with the necessary 
degree of exactness for this delicate purpose, has however 
hitherto put a stop to this enquiry ; and, as it is now rendered 
extremely probable that the parallax at least of the generality 
of stars is much below T, must continue to do so, until some 
very great improvement of micrometers shall enable us to 
measure tenths of seconds with as great certainty as we at 
present can units. 
I do not find that it has been noticed, however, that parallax 
