[Regulation 
of tne instru¬ 
ment. 
Laying a gun 
with, the tele¬ 
scope. 
Firing at an 
invisible 
object. 
Tests of 
accuracy. 
406 
LAYING GUNS BY TELESCOPE. 
TO REGULATE THE INSTRUMENT. 
It is necessary to regulate an instrument for use with each gun. 
For this purpose the gun is laid in the ordinary manner by a good 
marksman with the naked eye, on a very clearly defined object; the 
telescopic apparatus is fitted on the gun as described above, and the 
cross-hairs are moved by means of two mill-headed keys until their 
intersection is projected on the object. 
Mathematically, the axis of the telescope being 4 cm above the line of 
sight, the intersection of the cross-hairs should be directed on a point 
4 cm above the centre of the object aimed at. The optical axis of the 
telescope is then parallel to the line of sight, and will remain parallel 
to it as long as the relative positions of the telescope and its “ rest ” 
are unchanged, so that the instrument is regulated once for all for its 
own particular gun. All corrections in shooting must now be made 
with the tangent and deflection scales, without ever using the mill¬ 
headed keys, which should be replaced in the box as soon as the 
regulation has been completed. 
To lay a gun at any range—the proper elevation and deflection 
' having been given in the usual manner—the instrument is placed on 
the gun, which is then laid with the telescope instead of the ordinary 
sights. Cases may occur during a siege where it is necessary to direct 
a plunging fire on an invisible object. To use the telescope in cases of 
this kind it must be removed from its normal position on the “ rest,” 
and fixed on the other end so as to look backwards from the foresight, 
and act as a “ collimator.” 
It may be well to indicate a few experiments which appear to prove 
most conclusively the accuracy of the instrument* 
1. If a gun be laid with the telescope on the centre of a target one 
metre in diameter, at a range of 4000 or 5000 metres, the instrument 
may be taken off and replaced a great number of times, but the inter* 
section of the cross-hairs will always be found directed on the target* 
2. If a gun be laid with one of these instruments, which is then 
removed and replaced by another, the second telescope will be found 
to point in the same direction as the first, or, if not mathematically so, 
at least without any sensible deviation. One, therefore, acts as a check 
on the other. 
3. If one of these instruments be carried on a limber for several 
days over rough ground, and afterwards subjected to any of the fore¬ 
going tests, it will be found to give the same results as before. 
4. If a marksman lays 40 rounds in succession with the naked eye, 
after about 15 or 20 rounds his eye will begin to get dim, or at all 
events tired, whereas with the telescope he may go on laying the gun 
for five hours without inconvenience. 
5. In laying with the naked eye the deviations are 10 or 15 times 
greater than with the telescope, as I ascertained in the case of 156 
marksmen of the regiment. The results obtained in the 1st battery 
