495 
the parallax of «. Lyrce. 
The numbers in the same vertical column ought to have 
been equal had no derangements taken place. The index 
equation = — o",4 5, is stated to have been constant between 
July and November 1. Between November 1, and February 
25, it increased gradually to — s",6o. But the discordances 
in each of the vertical columns seem not to have relation to 
the changes of the index equation ; on the contrary the al- 
terations, that appear to have taken place, when the index 
equation is supposed to have remained the same, are as great 
as when it was changed. 
It may be said, if the relative positions of the microscopes 
remained the same for the day, no inconvenience could arise 
from their changing from one day to another. But what are 
the causes of these changes ? How can the accuracy of an 
instrument be relied on, or be estimated, that admits of such 
changes ? Besides, if we examine, the relative positions do 
not appear to remain the same for even a day, (2) (3) (4) 
exhibit the state of the differences for three several days : one 
in summer, one in autumn, and one in winter. Such discor- 
dances, it is true, are not found here as in ( 1 ) but they are 
much greater than could have been expected or ought to be. 
It may perhaps be supposed that these arise from errors of di- 
vision, but it is not likely that errors of division have any 
great influence. Indeed it is probable that this instrument is 
more accurately divided than any one that has ever been 
constructed. 
Table IV. This Table is constructed from observations of 
a Lyrse made with the Dublin circle. It exhibits the state of 
the side microscopes compared with the bottom microscope 
for about eight months (one season of the observations of 
* Lyrae.) 
