e, and many besides. Finally, Professor
Hussey's revision of the Pulkowa Catalogue[1604] is a work of the _teres
atque rotundus_ kind, which leaves little or nothing to be desired. The
methods employed in double-star determinations remain, at the beginning
of the twentieth century, essentially unchanged. The camera has scarcely
encroached upon this part of the micrometer's domain.[1605]

A research of striking merit into the origin of binary stars was
published in 1892 by Dr. T. J. J. See, in the form of an Inaugural
Dissertation for his doctor's degree in the University of Berlin. The
main result was to show the powerful effects of tidal friction in
prescribing the course of their development from double nebulae,
revolving almost in contact, to double suns, far apart, yet inseparable.
The high eccentricities of their eventual orbits were shown to result
necessarily from this mode of action, which must operate with enormous
strength on closely conjoined, nearly equal masses, such as the rapidly
revolving pairs disclosed by the spectroscope. That these are still in
an early stage of their life-history is probable in itself, and is
re-affirmed by the exceedingly small density indicated for eclipsing
stars by the ratio of phase-duration to period.

Stellar photometry, initiated by the elder Herschel, and provided with
exact methods by his son at the Cape, by Steinheil and Seidel at Munich,
has of late years assumed the importance of a separate department of
astronomical research. Two monumental works on the subject, compiled on
opposite sides of the Atlantic, were thus appropriately coupled in the
bestowal of the Royal Astronomical Society's Gold Medal in 1886. Harvard
College Observatory led the way under the able direction of Professor E.
C. Pickering. His photometric catalogue of 4,260 stars,[1606]
constructed from nearly 95,000 observations of light-intensity during
the years 1879-82, constitutes a record of incalculable value for the
detection and estimation of stellar variability. It was succeeded in
1885 by Professor Pritchard's "Uranometria Nova Oxoniensis," including
photometric determinations of the magnitude of all naked-eye stars, from
the pole to ten degrees south of the equator to the number of 2,784. The
instrument employed was the "wedge photometer," which measures
brightness by resistance to extinction. A wedge of neutral-tint glass,
accurately divided to scale, is placed in the path of the stellar rays,