ON SOME NEW TELEMETERS, OR RANGE-FINDERS. dll 
o!8 0? on a scale of 7 of the actual size. In this way two observers 
occupying only a very limited piece of ground may very rapidly deter- 
mine the data necessary for making a map of the whole tract of country 
visible from their station. 
The value of this method in military operations will be obvious. 
When troops are about to take up a new position, they may determine 
beforehand the distances of different landmarks from their intended 
position, so that the range of the enemy during action may be judged by 
reference to the points determined. This method of surveying would 
also, no doubt, prove very useful in exploring and prospecting, since the 
necessary instruments are very small and not liable to derangement, and 
in the ordinary practice of engineering surveying there are many cases in 
which it will be of value in enabling a survey to be made without travers- 
ing the ground to be surveyed. 
Of course a survey may be made in a similar manner by means of the 
fixed-angle telemeters referred to above, but the variable-angle instrament 
has the great advantage, even for this purpose, that the scale upon which 
the miniatures of the distances to be determined are set out can be varied 
at will to suit the nature and extent of the ground available for the 
observers. Besides this, the variable-angle instraments are much better 
suited, as has been said, for the rapid and accurate determination of 
direct distances. 
It is impossible to state what accuracy is attainable with these instru- 
ments, because that will obviously depend upon the nature of the object 
observed upon, the character of the light at the time of observation, the 
perfection or imperfection of the eyesight of the observers, the steadiness 
of the observers, and other elements; but we may say that it is easy to 
construct the instruments once for all to have in themselves no error that 
could be visible under the most favourable circumstances in the field, and 
this condition will be permanent. We have devised special means for 
measuring (in the workshop) the angles of reflection of the reflecting 
prisms and the angles of refraction of the refracting prisms to any desired 
degree of accuracy (to a few seconds of angle), and of adjusting the in- 
struments to give true readings without any field trials. These appliances 
we hope.on a future occasion to bring before the Association. 
Roughly speaking, it is easy to determine rapidly the distance of 
tolerably well-defined objects by means of the first or third instrument 
described to 10 yards at 1,000, which is, of course, well within any 
requirements for military purposes ; while for surveying purposes, with a 
little more time and care, and by taking the mean of several observations, 
a distance of about 1,000 yards can be determined within two or three 
yards with certainty, and a distance of 500 yards could be determined to 
one yard. As we have explained, the second instrument described 
(fig. 13) is not designed for the determination of distances with minute 
accuracy, but it may be used very rapidly, and gives very reliable results 
well within the allowable limits for infantry purposes. ‘The instruments 
are so simple in operation that we have found that a few minutes’ practice 
will enable an observer—previously ignorant of range-finding—to take 
quite good observations. 
In conclusion we would point out that the vital requisites of a military 
range-finder are that it should be incapable of derangement however 
rough the treatment, and at the same time that it should possess the 
facility of operation characteristic of constant-base instruments. Instrn- 
