1822.3 Memoirs of the Astronomical Society, Vol. I. 225 
line itself, which may always be made so by turning round the 
rism. While the image of the vertical line was separated from 
the line itself to its greatest distance, by turning the prism, the 
image of the star circulated round the star the space of an exact 
quadrant. In this situation the horizontal line and its image 
coincided as to breadth, but not as to length, just as the vertical 
line and its image had done before: and separating them to 
their greatest distance, brought the vertical line and its image 
again into a state of coimcidence; while the image of the star 
moved through another quadrant. The same appearances took 
place in the quadrantal point ofthe other semicircle. This expe- 
riment led to an immediate conclusion, that if a vernier connected 
with the revolving prismatic solid, were made to travel along a 
graduated circle, until a pair of stars and their images. are all 
seen arranged in one straight line, it would indicate, in that posi- 
tion, the angle that this line makes with the vertical or horizon- 
tal line, accordingly as the graduations might be figured on the 
limb; provided that the zero of the circle has been previously 
adjusted to the vernier, while one of the vertical or horizontal 
lines had its respective image coincident therewith.” The 
instrument was completely adapted to its purpose by substituting 
for the thread of a spider’s web, which was liable to be injured 
by turning the tube or cleaning the lens, a fine line made by 
drawing a diamond diametrically across the plain face of the 
lens. 
The next memoir contains ‘‘ Observations on the best Mode 
of examining the Double or Compound Stars ; together with a 
Catalogue of those whose Places have been identified, by James 
South, Esq. FRS. FLS. &c.” In consequence of Sir ae 
Herschell having employed in the examination of double stars, 
instruments of powers much greater than fixed instruments 
generally possess, a method has been given by him for finding a 
double star, not only equal, but indeed as he asserts, superior to 
having its right ascension and declination given. At the time 
when this opinion was expressed, and for several years after- 
wards, the highest power belonging to any of the fixed instru- 
ments did not exceed 80: they were, therefore, quite inadequate 
to this particular species of astronomical research. Now, 
however, it appears that telescopes admitting of magnifying 
powers equal to 500 or GUO are attached to fixed instruments ; 
and in all cases where compound stars can be resolved by such 
owers, Sir William Herschell, it may be inferred notwithstand- 
ing all that he has said apparently to the contrary, would fully 
admit their convenience and sufficiency. The difference, there- 
fore, betwixt the author and this celebrated astronomer is a mere 
shadow; and did not demand in its delineation any of those 
expressions of diffidence, which deference to an authority so 
elevated, if actually opposed, would inevitably call forth. ‘The 
peculiar fitness of fixed instruments,” says Mr. South, “ may be 
New Series, vou. 1v. Q 
