A. A. Michelson — Velocity of Lnght. 391 
image lasts, in this case, while the first image is sweeping over 
the face of the mirror, M. Hence, the greater the distance, RM, 
the larger must be the mirror, in order that the same quantity 
of light may be preserved, and its dimensions would soon be- 
come inordinate. The difficulty was partly met by Foucault, 
by using five concave reflectors instead of one; but even then 
the greatest distance he found it practicable to use was only 
_ for light to travel twice the distance between the mirrors. 
his displacement is measured by its arc, or rather, by its 
make thi 
mirror to the slit, and the speed of rotation should be made as 
great as possible. Se 
e second condition conflicts with the first, for the “radius 
the “distance,” therefore, the smaller will be the “radius.” 
There are two ways of solving the difficulty: first, by using a 
lens of great focal length, and, secondly, by placing the revoly- 
ing mirror within the principal focus of the lens. Both means 
h of the lens was 150 feet, and 
the mirror was placed fifteen feet within the principal focus. 
A limit is soon reached, however, for the quantity of light 
received diminishes very rapidly as the revolving murror ap- 
The chief objection urged in reference to the experiments 
tions of lenses and 
be relied upon within less than one per cent. 
fol riments the distance between the 
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