100 G. H. KNIBBS. 
In order to check the values assigned by the maker for the 
distances between the knife-edges of the rotation prisms, they 
were carefully measured by means of a Brown and Sharpe Manu- 
facturing Company’s micrometer gauge, with vernier reading. The 
results compare very well with those given by Bohme, the manu- 
facturer, as the following schedule of the measurements will shew. 
Distances between knife-edges. 
No.1 0:17875in. = 45402 mm. By Béhme 4:5403 mm. 
No. 2. 0:17875 4:5402 4°5396 
No. 3 0:17870 45389 45415 
No. 4 0:17875 45402 4:5393 
Mean By me 45399 4°5402 
If the mean result, say 45400 mm., be accepted as the true 
value of each prism, the greatest error arising from such an 
assumption will be only about one four-thousandth, which may be 
regarded as quite negligible. The distances 1135-0 mm., and 
2270-0 mm., are respectively 250 and 500 times the width between 
the knife-edges, or Z = 2501 in the first instance, and 5097 in the 
second; and since the doubles of these distances lie within the 
focal limits of the telescope, by means of which the reflection of 
the scale is seen in the mirror, they are suitable as standard 
distances to set out between mirror and scale when placing the 
instrument into position for an observation. The scale, graduated 
in millimetres, ranges between — 50 and +500, and is numbered 
+ Sei 
5, 4, 3,...1, 0, 1, 2,...49, 50. At the shorter standard distance, — 
viz., 1135 mm., the maximum value of 2 will be about 23° 46’ 30° 
—or more accurately 23° 46’ 10’—so that w will be about 11° 53’. 
From these data the following tables of corrections, conformably 
to formulz (6) and (7), have been computed by means of formula 
(4). The results are given to two places of decimals of a milli- 
metre in order to facilitate the formation by interpolations of a 
more extended table of corrections for practical use. In the work- a 
ing tables, the corrections would be expressed to the nearest tenth 
of a millimetre, since that is the unit obtained by estimation, 2 
reading in the mirror the reflection of the scale; and since also 
