APPENDIX NO. 8. TEST OF A TRANSIT MICROMETER. 
457 
1889,* that no clockwork was required, and that very good results could be secured 
with a hand-driven transit micrometer. He then constructed such a micrometer with 
which excellent results were secured. 
The Prussian Geodetic Institute made a series of tests of the Repsold transit 
micrometer in 1891, which showed that the apparent differences of the personal equation 
of the four observers who took part in the test were about one-tenth as large with the 
transit micrometer as with fixed lines and a key. The tests also showed that there are 
other well-marked advantages of the transit micrometer over other methods of observing 
star transits. 
The Prussian Geodetic Institute acquired two transits of the broken telescope type 
made by Bamberg, of Berlin, which were equipped with transit micrometers slightly 
modified from the Repsold design, especially as to the contact mechanism. From 1891 
to the present time the Geodetic Institute has had these instruments in use on various 
occasions on telegraphic longitude determinations. Its experience constitutes a strong 
proof that the use of the transit micrometer is effective in increasing the accuracy of 
such determinations. The latest work of this kind of which the results have been 
published at the date of this writing, is the determination in 1903 of the difference of 
longitude between Potsdam and Greenwich. 
The transit micrometer has been tried at various times in fixed observatories, and 
various opinions in regard to it have been formed. 
The consensus of opinion as published is strongly in favor of the transit micrometer 
as being effective in eliminating the effects of personal equation. 
In the United States the transit micrometer has been used continuously for several 
years at the Washburn Observatory at Madison, Wis. It has also been used at the 
observatory at the University of Michigan, Ann Arbor, Mich. Both of these instru 
ments are hand-driven Repsold transit micrometers. At the Philadelphia Central High 
School experiments have been made by Prof. M. B. Snyder with a transit micrometer 
driven by an electric motor. 
A short summary of the literature which is the basis for the above outline of the 
history of the transit micrometer is given in the latter part of this Appendix. 
When the question of using transit micrometers in making telegraphic longitude 
observations was under consideration in the Coast and Geodetic Survey, the facts, very 
brief!}’ outlined above, were available. While these seemed to be sufficient to leave no 
doubt that on large instruments used in fixed observatories the transit micrometer is a 
complete success, and that similarly it has proved successful when applied to longitude 
transits of the Prussian Geodetic Institute, it did not seem safe to predict the degree of 
accuracy which would be attained with transit micrometers applied to the particular 
type of transits used for longitude determinations in the Coast and Geodetic Survey. 
These transits carry a straight telescope, and therefore do not have the eye end of the 
telescope supported so rigidly as it is in the Geodetic Institute instruments, which have 
the eyepiece in the end of the horizontal axis of the telescope, and therefore quite 
near to one of the fixed points of support. The degree of stability of the eye end of 
the telescope is important with hand-driven micrometers, since the observations must be 
*Mr. F. D. Granger, Assistant, Coast and Geodetic Survey, had made the suggestion verbally in 
connection with longitude determinations in 1878; but his suggestion was not acted upon and never 
appeared in print. 
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