President's Address lili 
of the correlated constants, the results are to be combined by the 
method of least squares, having regard to the weight of each determina- 
tion, and with the further condition that the harmonic relations 
between the final values which theory reyuires shall also be rigorously 
satisfied. 
Keeping this principle in mind, let us examine what are the astro- 
nomical constants which, in the present state of the science, require 
determination to complete the solution of our problem. 
In the presidential address of 1880 I referred at length to the pro- 
mising method of the parallactic inequality of the moon’s motion. This 
inequality bears a well-defined mathematical relation to the solar 
parallax. The factor is a large one, but the unfavourable conditions 
under which the observations have to be made, and which render it 
unfit (in its hitherto employed form) for our purpose, I need not here 
repeat. 
It seems probable, from an admirable effort recently made by Batter- 
man, that if, instead of observing transits of the moon’s limb, the 
method of occultations of stars, and their reappearance at the dark 
limb, be employed, a valuable result may be arrived at. But to attain 
a full value such research must be accompanied by a complete discus- 
sion of the lunar theory, and a redetermination of the constants of that 
theory. This implies an immense labour, which can only be under- 
taken by a master mind entirely engrossed in such pursuits, and with 
a computing staff sufficient for the utilization of all the existing data. 
Fortunately we have in Professor Simon Newcomb a man having all these 
necessary qualifications, who, amongst other similar gigantic works, 
has undertaken to base the determination of new lunar tables upon all 
known_observations of occultations. When this has been done, there 
will doubtless result inter alia a valuable determination of the paral- 
lactic inequality and of the lunar parallax. This special question is, 
therefore, not a form of research demanding other effort. The Cape 
Observatory has, however, contributed a long list of observed occulta- 
tions, and continues still to supply them, to be utilized in Professor 
Newcomb’s great work. 
If we admit the entrance of purely physical researches into our 
problem, there is a most powerful method available for determining the 
solar parallax. The constant of aberration is an important astro- 
nomical one, having an annual period affecting the positions of the 
fixed stars, and as such capable of determination by purely astronomical 
methods. On the other hand, it expresses the ratio of the mean 
velocity of the earth in its orbit to the velocity of light. If, then, the 
velocity of light can be determined by direct experiment, and if that 
velocity is identical with, or bears a known ratio to, the velocity of 
