transit instrument at the Cambridge Observatory, 425 
The old methods of adjusting a transit instrument do not 
rest on such results ; and the old method of proceeding seems 
to me the most sensible one, that of separately and succes- 
sively correcting each cause of defective adjustment. 
The axis can be made horizontal, or its defect of horizon- 
tality known, by the level, the plumb line, or by reflection. 
The line of collimation can be adjusted by means of a small 
object in, or near to, the horizon. In this operation a small 
defect in the horizontality of the axis will have scarcely any 
effect on the accuracy of the operation. If the mark should, 
for instance, be 2° above the horizon, and one end of the 
axis s" higher than the other, the error in collimating from 
that cause would, in the latitude of Cambridge, be only 
o".i075. The error in the same operation with the pole 
star, supposing it be fixed, would be 1' 11". 5. 
The third adjustment, which is the most troublesome, is to 
place the transit instrument in the plane of the meridian ; and 
there are two methods of effecting this : one, by high and low 
stars ; the other, by circumpolar stars, or, as it almost always 
happens in practice, by the pole star. 
The essential difference in these two methods is, that the 
former rests on the results of astronomical science, whilst the 
latter does not so rest ; and this circumstance gives the latter 
a decided advantage over the former, when it is necessary to 
make a nice adjustment. Yet there is not wanting consider- 
able astronomical authority for placing the two methods on a 
level, the one with the other. Baron de Zach, for instance, 
views each as an equally good method ; and in his Tabulae 
speciales Aberr®. et Nut*, gives instances of the adjustment 
of a transit instrument ( a 5-feet one by Dollond ) ; the first, 
by the comparison of the passages over the meridian of 
