220 
sion, are so arranged that one group culminates 
in the evening and another in the morning 
hours. The range of the variation in latitude, 
given in the final corrected results, is 0/7.44, 
and the probable error of a single determina- 
tion is 0.134. Two maxima and minima are 
covered in the period of observations, the range 
of the maximum and minimum in the middle 
of the series being somewhat less than the ex- 
treme range. Among the interesting details 
brought out in the critical discussion are: that 
the value of the micrometer screw is variable 
from other causes than change of temperature ; 
that the deduced constant of aberration is un- 
expectedly large, viz., 20’7.580, and that the 
same pairs of stars gave results on different 
nights which differed occasionally by many 
times the computed probable error of the ob- 
servation. No satisfactory explanation of this 
anomaly has been found. The corresponding 
anomaly in longitude work is plausibly ex- 
plained by variation in personal equation of the 
observers, but it is not easy to apply this ex- 
planation to zenith telescope observations. 
CHAMBERLIN OBSERVATORY OF THE UNI- 
VERSITY OF DENVER. 
PROFESSOR HOWE continues to make micro- 
‘metric observations of superior excellence with 
the Bruce micrometer attached to the 20-inch 
equatorial. His careful work upon the fainter 
nebulz has been embodied in two communica- 
tions to the Royal Astronomical Society which 
are published in Monthly Notices, Vol. LVIIL., 
Nos. 6 and 9. The extensive series of observa- 
tions of Eros from September 12, 1898, to April 
6, 1899, is given in the Astronomical Journal, No. 
463. 
ASTRONOMICAL OBJECTIVES. 
ZEIss’s new catalogue calls attention to the 
improvements in objectives which have resulted 
from recent studies encouraged by the varieties 
of glass manufactured at Jena. The price lists 
include six or more special combinations having 
individual excellences. The binary apochro- 
matic lens with ratio of aperture to focal length 
1:17 to 1:20 nearly eliminates the secondary 
spectrum, as does Ko6nig’s combination of one 
flint and two crown of shorter focus (ratio 1:10 
to 1:15) which is styled the triple apochromatic 
SCIENCE. 
[N. S. Vou. X. No. 242. 
lens. The ordinary silicate glasses are used as 
heretofore with long focus (ratio 1:15 to 1:18) 
by Fraunhofer’s formula, and there is also a 
lens of longer focus especially adapted for astro- 
photographic purposes. Short-focus lenses are 
represented by a triple lens with ratio 1:4 to 1:6 
designed for finders and a binary lens with flat 
field, both of which have an uncorrected sec- 
ondary spectrum. It would be interesting to 
_ learn how many of these special combinations 
will come into actual use. 
WINSLow Upron. 
PROVIDENCE, R. I., 
August 10, 1899. 
NOTES ON PHYSICS. 
THE London Electrician states that on July 
3d Lord Kelvin sent to the Royal Society the 
following in a telegram: ‘‘ An electrified body 
is set into rotation by the generation of a mag- 
netic field around it. The magneto-optical 
phenomena discovered by Faraday, Kerr and 
Zeeman are to be thus explained.’’ In the 
next issue of the paper a letter from Lord Kel- 
vin states that this announcement was not based 
on experimental results, but deduced from the 
current that flows in a metallic conductor at 
right angles to a growing magnetic field. The 
telegram is discussed editorially and in a letter 
by G. F. Fitzgerald. The point seems to be 
that the growth of the field will cause a dis- 
placement of the charge round the body which 
will be the equivalent of a momentary current, 
and that this reacting on the field will cause ro- 
tation, which, if the body is frictionless, will 
continue till the stopping of the field produces 
an equal opposite torque. Fitzgerald states 
that he has considered this matter, but doubts 
if the forces will be great enough to permit of 
experimental demonstration. In the same issue, 
and in connection with this matter, S. P. 
Thomson describes and discusses a phenomenon 
presented to the Royal Society by C. E. S. 
Phillips, in which a vacuum tube having iron 
electrodes which can be powerfully energized 
by an external electromagnet is used. When 
a discharge has been sent through the tube, and 
then cut off, even for so long as ten minutes, 
and then the magnets are energized, a brightly 
luminous ring forms normal to the field, and 
