506 REPoRT—1890. 
Fig. 10 is a side view of the chief observer’s instrument as seen by the 
assistant observer. The upper part of the plate upon which the mark m 
is carried is hinged to the lower part for convenience in packing. Fig. 11 
shows a sectional elevation, and fig. 12 a sectional 
Fig. 10. plan of the chief observer’s instrument, P being 
mw the reflecting prism and Rk the refracting prism. 
The latter is shown as constructed of crown and 
flint glass, so as to be achromatic. This is not 
necessary except for telescopic observation, but 
it possesses the advantage that, by rotation of the 
crown and the flint relatively to each other, the 
angle of deviation of the compound prism can be 
varied within certain limits, and so adjusted to a 
desired angle. The reflecting prism P is placed 
above the level of the centre of the instrument, 
so that a direct view is obtained below it through 
the refracting prism r. The prism P is rigidly 
fixed in the frame F by means of a very hard 
cement—almost as hard as the glass itself—so 
that relative rotation of the prism and the frame 
is absolutely impossible. The prism Rk is similarly 
rigidly fixed into the carrier c, a flat portion 
being ground off the circular prism, and its place 
in the carrier filled by a metal sector soldered 
in. The carrier ¢ is supported in F so as to be free to rotate, and it is 
milled on the edge p to facilitate the rotation. s is the scale ring, 
graduated on the periphery, as above described; and> indexes at 11 
serve to read the scales for the 25-yard base and the 50-yard base re- 
spectively. kK is a piece which covers the scale except in the vicinity of 
the indexes. 
The construction of the frame F and carrier ¢ is such that there are 
no openings by which dust or rain can get into the interior except 
through the small eye-hole B, and even this can be closed if thought 
desirable by a small piece of glass. The plate upon which the mark mu 
is carried, together with the tubular piece tT to which it is fixed, may be 
removed by unscrewing the eye-tube u, so that more convenient access 
can be had to clean the outer face of the prism Pp, for which purpose, also, 
the frame r is bevelled off at B. 
It need hardly be pointed out that the angle of deviation of the prism 
R cannot be altered by accident or by use, neither can the angle set out 
by the prism P be altered. Moreover the prism Rk is rigidly fixed to the 
scale-piece, so that relative motion is quite impossible ; and the prism P is 
fixed into the frame Fr, upon which the indexes are marked, so that 
relative rotation between these is impossible. Provision is made for the 
removal of the prism-carrier c from its bearing in the frame F at an 
time for cleaning, and its replacement, without of course disturbing the 
attachments of the prisms to these pieces. In fact the whole instrument — 
can be taken to pieces in a few seconds (if, say, it has been under water), 
and half a minute will suffice to clean the faces of the prisms, so that 
within less than a minute the instrument can be taken to pieces, cleaned, 
and put together again without the remotest possibility of anything being 
put out of adjustment. It will therefore be seen that the instrument is 
incapable of suffering any optical derangement from accident or USC, — 
Pio eh eesnsesome- 
