parallax of certain fixed stars. 301 
surprise at the erroneous results of Hooke's observations ; 
which he, Dr. Bradley, had considered as exact, till they were 
contradicted by Mr. Molyneaux’s observations, and by his 
own. He says, “ I cannot well conceive that an instrument of 
the length of 36 feet, constructed in the manner he describes 
his, could have been liable to an error of 30" (which was doubt- 
less the case), if rectified with so much care as he represents." 
It may be remarked here, that the results of the observa- 
tions of a. Cygni with the fixed telescope of Mr. Pond, as 
given in the last volume of the Transactions, would in them- 
selves appear to indicate a considerable parallax in that star, 
and thus produce an error similar to that of Hooke. But 
Mr. Pond guarded against such a source of error, by using 
two stars ; and therefore no derangement of the instrument 
could affect his results, except as far as it might take place 
between two succeeding observations. 
Flamstead's instrument, which he has described in his 
letter to Dr. Wallis,* was a mural arch of 140°, by which 
he could observe all stars visible in his hemisphere ; and 
observe below the pole all circumpolar stars that were not 
above ii°i-from the pole. He deduced the index error of 
his instrument by observations of the pole star corrected for 
refraction ; not at first considering any correction for paral- 
lax as necessary : but he soon found a correction necessary, 
which he attributed to the effect of parallax. In this he 
was singularly mistaken. The result of his observations 
was, that the diameter of the circle described by the pole star 
about the pole, was Tao", or T30" greater in summer than 
in winter. 
Wallis Oper. tom. iii. p. 701. 
