A post regarding a Barr & Stroud Range finder prompted some research into this interesting piece of equipment, Used by both the Artillery and Infantry to determine ranges to targets.

Barr & Stroud Range finder (Front View)

Barr & Stroud Range finder (Rear View)

The principle is the instrument formed a fixed length base with prisms at each end picking up the target, so effectively forming two triangles, allowing trigonometrical principles to determine the range. One eyepiece was used to line up the target picture which each showed half of the picture. By lining the images up (coincidence) , the range could be read off in the second eyepiece.

Barr & Stroud Range finder Principles

Barr & Stroud Eyepiece Coincidence of Targets

HANDBOOK OF ARTILLERY INSTRUMENTS 1914

Length: over all 44 1/4 inches. Base length, 1 metre (39.3 inches)

Diameter of body, 3 inches

Maximum diameter (over end caps), 5 1/2 inches.

Weight, 13 1/2 lbs

Range scale graduated from 500 to 20,000 yards.

Magnification of Telescope X 13.

Field of view, horizontal 3 degrees 10 minutes

Field of view, vertical 2 degrees 40 minutes

Eyepieces inclined to the horizontal at 60 degrees

GARRISON ARTILLERY TRAINING Vol 1 1914

Sct 126 The Barr and Strange Range Finder

The instrument in a "one observer"instrument with

horizontal base, and depends in principle on rays reflected

through prisms at either end of the tabs which forms the

base.

Advantages

It is portable and simple to work. It measures ranges from a fixed horizontal base and can there-

fore be used at any height and requires no leveling or datuming points. It is a one-observer instrument, therefore

errors inherent in many horizontal base instruments are eliminated. It is very simple and easy to learn. It has no

micrometer screws or easily wearing delicate parts. It does not easily get out of order and can be readily re-adjusted.

Range finding with it is easy and accurate at night as in the day time. When properly adjusted it will work to

an accuracy of nearly 98 percent at 5,000 yards range, and in this respect compares favorably with D.R.F

and depression P.F. from heights below 50 feet

Disadvantages

It is only accurate up to medium range. Gun differences will be as necessary as with D.R.F., hence it

will be desirable to keep the instrument close to the battery,involving risk of damage, &c.

Barr & Stroud Range finder Think these chaps might be infantry