390 Chronicles of Science. [ July, 
it appears that, with the double-image micrometer, errors are to 
be feared arising from the following sources :~-(1), Periodical 
errors of the micrometer screw; (2), Variability in the mutual 
distance of the threads of that screw; (3), Distortion of the images. 
The periodical errors of the screw may be very different at small 
differences in the readings of the micrometer-head, and require 
therefore a particular inquiry. The errors arising from the sources 
(2) and (3) do not compensate themselves in brief periods, and 
their common amount may be determined by the same inquiry. 
In the micrometer as made by Simms one half-lens is quite fixed, 
while only the other can be moved by the micrometer-screw. This 
construction makes it impossible to eliminate the periodical errors of 
the screw at the measurements themselves. If the fixed half-lens 
could only be moved so much as the amount of one revolution of 
the micrometer-screw, we could hereby (according to Bessel’s 
theory) eliminate at each measurement the periodical errors of the 
screw, and this would be a great gain. If the fixed half-lens could 
be moved as much as the movable one, the measurements could 
be extended to angles twice as large as can now be measured with 
the micrometer. Then it would, however, be shown together, with 
relation to the fixed lenses. By this modification the price of the 
instrument would certainly be considerably raised, but it would be 
well worth the extra expense. 
After a long and elaborate investigation of the various errors 
and methods of eliminating their influence on the final results, 
Prof. Kaiser arrives at the conclusion that the Astronomer Royal 
has, by the invention of his double-image micrometer, rendered an 
important service to astronomy. Indeed, he doubts whether, besides 
the heliometer, a second double-image micrometer exists by which 
measurements can be effected so accurately as by Airy’s instrument. 
That instrument requires, however, a very rigorous and difficult 
inquiry, in order to give it the accuracy of which it is capable, 
and it demands also great prudence in the use of it. 
A micrometer like Airy’s can only render its services if it is 
adapted to a large and precious refractor. The price of the 
micrometer would, even if it were doubled, still remaim very 
insignificant in comparison with that of the refractor. Therefore 
it would appear very desirable to give to the micrometer the more 
complicated construction above alluded to, even if its price were 
thereby considerably increased. 
Rey. T. W. Webb has given a further notice of the great 
nebula in Orion. A comparison of the various representations of 
the great nebula in Orion, which have been given for a period of 
many years, seems to lead inevitably to the conclusion that our 
knowledge of its real aspect is still far from complete. While Mr. 
Huggins’s most important discovery of its true constitution renders 
