334 
LORD WROTTESLEY ON THE PERIODICAL OBSERVATION OF STARS 
In conducting the observations, however, difficulties arose that had not been anti- 
cipated: Sir John Herschel’s lists contain sixty-nine stars; many of these, at the 
periods announced in the lists as proper for observation, if observed at all, must be 
observed at distances from the meridian too great, when the delicate nature of the 
inquiry is considered ; many of them, having south or small northern declinations, 
are near the horizon when the proper period for their observation arrives. Another 
very serious impediment existed in the obligation imposed of obtaining half of the 
measures in the early morning hours ; the observer, if employed in the observatory 
on the preceding night, was sometimes fatigued and unequal to the task ; heavy fogs 
frequently came on at that time, and an enormous deposit of dew on the interior sur- 
face of the object-glass often seriously incommoded the observer ; an evil which, 
until an opening was made in the tube near it, was irremediable, since the use of 
dew-tubes failed in preventing it. 
To these and other causes it is owing, that after more than six years devoted to 
this course of observation, I am compelled to apologise for the meagre results which, 
for reasons about to be mentioned, I have still ventured to lay before the Royal 
Society in their present shape. Of sixty-nine stars I have only obtained observations 
of forty-eight, and of these forty-eight, twenty-nine have only been observed at one 
period of the year. 
It is a most discouraging feature in this class of observations, that, however nume- 
rous and trustworthy the measures obtained at one period of the year may be, there 
may still be a failure to procure, at the expiration of six months from their date, 
measures worthy to be compared with them ; and the function of the parallax, being 
the difference between the two results, the value of that element is of course affected 
by the whole amount of error with which either result is charged. 
It has often happened that a star has been observed at one of the assigned periods, 
and that no corresponding observations have been procured at the expiration of six 
months, or the next succeeding period ; thus, for example, calling the first epoch in 
the year at which the star is marked for observation, the early, and the second the 
late period, a star has been observed two or more times successively at the early 
period, when no corresponding observations could be procured at the following late 
period. 
The question then naturally occurs, whether the communication of the measures 
actually obtained may not be deemed premature. I consider, however, that I have 
proceeded sufficiently far to demonstrate the impolicy of further perseverance, with 
the means at my command ; the rather that instruments are now erected both at Liver- 
pool and Oxford, which are pre-eminently suited to this class of observations, and 
therefore it would be only a waste of time and force, which may be more profitably 
employed in other ways, to devote any further attention to the inquiry. 
I proceed, therefore, without further apology, to describe the means employed, the 
mode of employing them, and the results obtained. 
