Progress of Astronomy during the Year 1879.- 473 
out too much diiticulty, a direct-vision prism has been inserted 
between the object-glass and the eyepiece, and each nebula 
examined through it. If the spectrum is continuous, its light will 
be spread out into a luminous band. 
Part III. of the “ Astronomische Beobachtungen auf der Gross- 
herzoglichen Sternwarte zu Mannheim” contains micrometrical 
observations by Dr. Valentiner of the clusters G. C. 4410 (VIII. 
72), and G. C. 1166 (M. 36). The stars were observed by means 
of differences of R. A. and declination. Both these clusters are 
rather loose ones, still measures of these and similar groups are 
sure to become of great importance in future, especially photo- 
graphic records of them. 
14. Photometry. 
One of the most important publications of the year is the 
eleventh volume (Part I.) of the “Annals of the Observatory of 
Harvard College,” containing Professor Pickering’s Photometric 
Observations made with the 15-inch refractor. The author first 
made experiments with a Zéllner’s photometer and other instru- 
ments in which the star is compared with an artificial light, but 
on account of the want of similarity between the real and the 
artificial star, and the difficulty of applying a correction for 
changes in opacity of the air, which only affect the real star, the 
use of a lamp was abandoned, and all comparisons were made 
with some bright star in the vicinity of the object observed. All 
variations in the condition of the air were thus eliminated, both 
objects being equally affected by it. To insure a comparison free 
from personal equation or variations due to the observer, it is 
very desirable that the objects shall resemble each other as much 
as possible. This result is secured by bringing the images to be 
compared close together, so that they are both viewed with the 
same aperture and magnifying power, and the light of both is 
equally distorted by passing through the same lenses and prisms. 
A detailed description is given of all the various photometers 
employed.* The first instrument used was constructed by 
attaching a Nicol to a double-image prism in such a way that it 
could turn freely around its axis, while by a graduated circle 
* An abstract of this description is given in Nature, xxi., p. 23, 
