SHORT NOTES ON PROFESSIONAL SUBJECTS. 
5 
9. Small telemeter for military purposes.* This instrument consists 
of a strong tube about 2 feet in length; at each end there is a small mirror placed 
at an angle of 45° to the axis of the tube, and in the centre two other mirrors 
crossing each other at right angles. A telescope with micrometer eye-piece is fitted 
into the centre of the tube. 
Horizontal section of the Telemeter shewing the arrangement of the mirrors. 
Note. —There is an opening in the centre of the instrument for direct vision, and for the purpose 
of illuminating the micrometer wires when using the instrument at night. All openings can be 
closed with shutters when required. 
A prismatic compass is fixed on the top of the tube, so that the bearing, as well as 
distance, of any object can be at once determined. 
The image of any distant object is reflected from each mirror at the ends of the 
tube (or base-line), to the mirrors in the centre, and from the latter into the 
telescope, thus giving two images of the object observed. 
An opening is provided in the centre of the tube, above the mirrors, which enables 
an object to be seen by direct vision; this allows of the adjustment of each mirror 
to be made separately. 
The instrument having been once adjusted so that the mirrors are parallel to each 
other, needs no further adjustment, but any deviation from parallelism must 
be frequently ascertained and applied as an index correction to the observed 
reading. 
In this instrument the zero reading for 100 feet is 869, and any deviation from 
this is the index error. 
The value of 1 division of the micrometer head is 11 seconds of arc. 
This instrument will be found very useful for road surveying, as the distances can 
be determined with great accuracy up to 600 feet, and it can be used with tolerable 
certainty up to 8000 feet. To ensure correctness in the readings it must be 
carefully shaded from the direct action of the sun’s rays, as they cause an unequal 
expansion of the tube. 
A similar instrument, 6 feet in length, has been found to give good results up to 
3000 yards. 
Note. —Each observer must determine and apply his own index correction as it may vary with 
the focal length required by different eyes. 
2 
* According to a design of Col. Clerk, R.A., E.R.S. 
