Mr. HERSCHEL on the ’Parallax of the Fixed Stars. 
^ a 
made, fhould have an adequate power and difiin chiefs. Upon 
the firft head, the great improvements of mathematical inflru- 
ment-makers have hardly left us any thing to define : we can 
now meafure feconds with almoft as much facility and truth as 
former obfervers could meafure minutes ; nor do I think it im- 
poflible to go hill further, and divide inftruments that would 
fhew thirds with fufficient accuracy. It is in the latter, or 
optical part, we find the greateft difficulty. To lee a tingle 
fecond of a degree with precilion requires a telefcope of very 
great perfection ; therefore, fuppofing the mechanical part of 
an apparatus well executed, it will hill be necellary to try how 
far the power of our telefcope will enable us to afeertain with 
confidence the divifion or number of feconds it points out. If 
upon trial we find that our inftrument will give us the fame 
meafure within the fecond, every time the experiment is- re- 
peated, we may pronounce it capable of meafuring feconds ; 
if otherwife, it will remain to be examined, whether the fault 
lies in the mechanical or optical part. 
Let us now fuppofe that the parallax of the fixed flats does 
4 
not amount to a fingle fecond, yet flill the cafe is by no means 
defperate ; and though the difficulty of meafuring feconds will 
foon fuggeft to us what extraordinary powers and diftin chiefs of 
the telefcope, and accuracy of the micrometer, are required to 
meafure thirds ; this ought by no means to dilcourage us in 
the attempt. Could we meafure angles, much fmailer than 
feconds, might we not hope to find the parallax of fome of the 
fixed ftars at leaf to amount to feveral thirds ? On the other 
hand, if it fhould appear, indeed, that even with finch im- 
proved methods of ineafiurement -we could not reach the 
remote fituation of iuch almoft infinitely diftant funs, we 
anight flill derive a valuable approximation towards truth from 
M 2 jfuch 
