Progress of Astronomy during the Year 1879. 475 
The image of some bright star, assumed as standard, passes 
outside the telescope, and is reflected into the field, after having 
been reduced by some known amount, until it equals the faint 
object to be measured. 
The instruments having been described at length in Chapter I, 
the second chapter contains the journal of the observations, 
while the discussions are given in Chapters IIT and 1V. Con- 
Junctions of planets, already alluded to above, afforded a good 
opportunity for comparing their relative brightness. The results 
are :— 
Albedo of Saturn 
Albedo of Mars 
Albedo of Jupiter 
Albedo of Venus 
A series of photometric measures of all the more conspicuous 
double stars is discussed in Chapter IV. 
The recently published annual report of the Harvard College 
Observatory shows that these photometric researches are being 
continued with great energy. A circular has been issued to 
Astronomers about “Stellar Magnitudes.” In the hope of pro- 
viding at least a partial remedy for the confusion in estimations 
of faint stars which hitherto has existed, a series of photometric 
observations has been undertaken of stars of various magnitudes, 
situated near the North Pole (this being always a comeatible 
place for northern observers). The co-operation of astronomers 
in giving estimates of the magnitudes of these stars is requested. 
This undertaking is no doubt eminently useful, and may be ex- 
pected to lead to valuable results. The report further mentions 
that the light of nebulz is now also being measured. The com- 
parison is here made with the image of a star thrown out of 
focus by such an amount that it will appear of the same intrinsic 
brightness as the nebula. Knowing the stellar magnitude a 
simple computation serves to show what must be the magnitude 
of a star which, when spread over a circle one minute in 
diameter, would emit the same amount of light as an equal area 
of the nebula. The light of the nebule may accordingly be 
described in stellar magnitudes. 
A work of some magnitude has been undertaken by Professor 
Pickering during the past year in the determination of the hieht 
of all the stars visible to the naked eye at Cambridge (Mass.) 
= 4-62+007 
= 0-86+0-02, 
