2 Observations of Nebule and Clusters of Stars. 
The observations of the great nebula in Orion having been given at some length 
in a paper published in the Philosophical Transactions for 1868, together with an 
engraving which embodies all the features of that nebula as noted up to that date, 
the earlier observations of that object are entirely omitted here, as also those 
of the great nebula in Andromeda, which has been examined frequently, and 
micrometrical measures of some of the very numerous stars in it made, but we 
do not at present see our way to the execution of a drawing of this object 
exhibiting any new features of interest. 
The brighter and more striking objects of Sir John Herschel’s Slough-Catalogue 
(Phil. Trans. 1833) having been first examined, also the more interesting ones 
having been more or less carefully delineated and the drawings published, in the 
former papers,* there remained less scope for the pencil, and in place of searching 
for new examples of the spiral configuration first noticed in M. 51=h. 1622, with 
the six-foot instrument, and pushing further with the aid of freshiy and most 
carefully figured specula, the resolution of nebulze into clusters of stars, the 
micrometer has been more frequently used, and in this work the clock movement 
applied in the year 1868 has been found of great service. 
The micrometer is the same which has been used almost from the first. It was 
constructed by Mr. Thomas Grubb. It is fitted with two micrometer heads with 
screws of rapid pitch (five threads=one inch) to allow of its being used without 
clock movement, and with a thick position-wire and two distance-bars composed 
each of three fine wires stretched side by side, requiring no illumination, The 
errors of the screws and of the position circle were investigated by Dr. Copeland 
and Mr. Dreyer, and found to be exceedingly small. Another micrometer with 
fine lines for “bright wire illumination,” recently constructed for me by 
Mr. Howard Grubb, has not as yet been found so serviceable, but experiments 
with it have shown that bright wires may be used even on faint nebule, 
Owing to the peculiar construction of the universal joint which supports the 
six-foot instrument, the primary axis being fixed due east and west and horizontal, 
and the secondary at right angles to it and to the axis of the tube, an errort is 
introduced into the position angles, which for some time escaped notice, and it is 
* Philosophical Transactions for 1844, 1850, and 1861. 
+ The formula for the correction of this error is tang a = tang / cos A where / is the hour-angle, 
A the NPD of the object, and « the correction; x is negative west, positive east of the meridian for 
objects north of the equator, the reverse for southern objects; it is zero at the equator, and in other 
positions varies nearly as the hour-angle up to 23° near 70°, 54° near 50°, and 10° near 30° NPD, at the 
extreme distance from the meridian at which the micrometer has been used. 
The constancy of position of the axes of the joint was proved hy Dr. Copeland by means of a series of 
measures of the same pairs of stars, from the meridian to the extreme limit of range, which when corrected 
by the above formula were found to agree within the limits of the errors of observation. 
The measures were, in nearly every case, taken from the inner edges of the two bars. On employing 
test measures to ascertain if this method caused a systematic error, taking the same pair of stars and 
measuring their distance apart alternately, by both inner edges of the bars, and by both outer edges, no 
difference was perceived ; but in the case of a measure of distance between the dise of a planet and its 
satellite, a constant error more probably will arise. 
