100 
the case with other globular clusters. Surrounding the cluster are 
curves and festoons of stars, which is a characteristic of these objects. 
Eye observations do not reveal the existence of the involved nebu- 
losity which, on the plate, is sufficiently dense to obscure the stars, 
though they are visible through the nebulosity on the negative. 
The next photograph (PI. ii.) is of the cluster known as M. 13, 
in the constellation Hercules, and isin R. A. 16 h. 38 m., decl. N. 
36 deg. 39 m. The scale is 6 seconds of arc to 1 millimeter, and the 
field or circle is r8 minutes of arc in diameter. The photograph 
was taken with the twenty-inch reflector and an exposure of one hour, 
on May 22, 1887, and delineates one of the finest globular clusters 
in the heavens, containing thousands of «tars densely packed 
together at the centre and with curvilinear streams of stars radiating 
from it. Lord Rosse detected three dark lanes or rifts in its interior, 
forming something like the letter Y, which is distinctly shown on 
the photograph and more strikingly visible on the negative. No 
drawing can possibly do justice to an object like this, which is por- 
trayed by photography in one hour. Moreover, it shows the cluster 
involved in nebulosity obscuring the stars at the centre, a fact which 
observers had hitherto failed to perceive. 
The next photograph (Pl. iv.) is known as Herschel VI., No. 33 
and No. 34, in Perseus, having R. A. 2h. 11 m., decl. N. 56 deg. 
38m. The scale is 24 seconds of arc to 1 millimeter, and the pho- 
tograph covers the sky area of 1 deg. 54m. by 1 deg. 38m. It 
was taken with the twenty-inch reflector, on January 13, 1890, with 
an exposure of three hours. These gorgeous clusters, in the sword- 
hand of Perseus, reveal one of the most brilliant objects in the 
heavens. ‘To chart their component stars by eye observations and 
measurements would be an exceedingly protracted task, and even 
then it would only be imperfectly done. The photograph gives a 
perfectly accurate picture of these thousands of stars in a very short 
time, the relative position and magnitude of each one being cor- 
rectly delineated, so as to form a reliable basis for future investiga- 
tion concerning their variability and relative movements. 
Next (Pl. v.) is a photograph of the ring nebula, M. 57 Lyre. 
It isin R. A. 18 h:-49'm., decl-N. 32 deg. 52 m. The sealegaee 
seconds of arc to 1 millimeter, and the diameter of the field is 12 
minutes of arc. The photograph was taken July 27, 1891, with the 
twenty-inch reflector and an exposure of thirty minutes. The 
nebula is the best known and brightest of the class of annular 
