George Forbes’ rangefinder


The story of George Forbes’ rangefinder, and the circumstances of its development, offer a fascinating insight into the development of military technology at the dawn of the twentieth century.

By the time of the second Boer War (1899-1902), field and navel artillery had undergone a period of swift technological advance. In the 1880s navel artillery’s longest range was approximately three quarters of a mile. Thirty years later, that had increased to eight miles, and by 1918, Imperial Germany’s ‘Paris gun’ had a range of 81 miles.

Vital to the operation of such weaponry is a rangefinder, a device to calculate the distance between the user and a distant object or landmark. Without this, projectiles are destined to over or undershoot.

The British military was deeply unsettled by the war in South Africa – an unfamiliar, guerrilla conflict of movement. German technological advance was no less disturbing. Rangefinders were just one of the areas where the War Office sought innovation.

As early as 1894 the shortcomings of the British military’s existing rangefinder, the Mekonometer, were well-known. Major Willoughby Verner of the Rifle Brigade told the Royal United Service Institution (RUSI) in a talk that year that the British device was ‘woefully inferior’ to its French and Russian counterparts. During the second Boer War it was common for officers to buy, from their own pockets, expensive foreign-made, rangefinders. 

For some years different branches of the services divided in their approaches to this technology. Navel use tended towards huge fixed rangefinders that required two operatives to take sightings from perhaps two meters apart. These provided measurements from which distance could be calculated using trigonometry. Artillery also tended to larger, albeit mobile, equipment. The infantry did not consider such devices an option.

As early as 1902, Forbes described his ideas for a rangefinder in the journal The Scientific American. In this he wrote that he had worked on the project intermittently for a decade and intensively for two years. His objective was to create a light-weight rangefinder for use by soldiers on the move. At the time of that article, he clearly had a prototype. An account of the British Association’s annual meeting in Belfast that summer noted that: “an account by Professor George Forbes of the practical trials in the South African War of his beautiful range finder excited great interest.” In the same year, the Society of Arts awarded his paper on rangefinders a silver medal.

His personal involvement in fighting during the Russo-Turkish War almost certainly informed his sense of what would be most useful to soldiers contesting open countryside.

Forbes’ rangefinder, sold by Lyon & Turnbul (Edinburgh) for £225 in 2017

One issue for such users was the requirement to estimate the distance of indistinct objects. Church steeples and warships on the horizon are significantly easier to ‘distance’ than khaki-clad soldiers traversing a landscape, Forbes observed.  

In 1902 he was one of three developers who agreed to submit their devices for trials by the War Office. 

The others were Barr & Stroud, the Glasgow-based precision optics manufacturer, and Captain A H Marindin, an infantry officer who had experimented privately with rangefinder designs since 1895 and whose designs were manufactured by Adam Hilger & Co of London.

By this time, Barr & Stroud were supplying the navy with rangefinders and were producing a hand-held model that was well considered. Forbes, however, appeared to have the initiative. He made presentations at RUSI, and the Royal Society of the Arts and obtained endorsements for his design from, among others, Lord Kitchener. 

According to Stephen Sambrook in The Optical Munitions Industry in Great Britain, 1888–1923, William Stroud became fearful that Forbes would pip them at the post. At one point he considered a suggestion from Forbes that his company concentrate on the navel market, and not compete in the artillery market. In another, Stroud contemplated creating a product that emulated many features of Forbes’ design.

Forbes patented his rangefinder in 1903 and appears to have had several prototypes built, by Zeiss in Jena, Germany. The one pictured is approximately 20 cm x 20 cm x 10 cm at its broadest dimensions. From memory the one I tried weighed a shade over a kilogram – pretty hefty for a hand-held instrument, but similar in size and weight of quality pair of binoculars of the period.

It seems likely that Forbes created more than one working design, as some of his descriptions include a fixed base for the device. Of the two that I have seen, neither included this feature.

To make his device work, Forbes relied upon his pre-existing knowledge of the refraction of light and the way that it distorts over the distances over which a rangefinder might be used. He made calculations also based on the speed at which the muscles in the eye learn to compensate for incongruous sights and made serious-minded use of stereoscopy, a technology that is best know in ‘parlour-use’ devices that allow users to have the impression of observing a three-dimensional scene. The adjustment knobs on Forbes’ device required the precision of a micrometer it they were to be effective.

Looking through the eyepiece of the model that I tested some years ago, the immediate impression was of looking through a pair of binoculars. 

However, there are two, small, etched images  of a Scots soldier in a hot air balloon in the middle of the field of view of each eye. The device is operated by ‘placing’ the ballon over the object whose distance you wish to calculate. Then, by turning the knob to the side of the eyepieces it is possible to cause the two images to become closer and eventually to sit atop one another and apparently become one. At this point, the distance to the object in question can be read off the dial in yards.

The ‘balloon’ focusing object appears whimsical. Forbes gave a considered reason for its use, however in his Scientific American article.

The only real object I could think of with a point lower end of which is at the center of the field was a balloon with a tail rope. This has been a grand success, and the two balloons spring together even with observers who could never bring the lines to look like a pole at a fixed distance. Al­most everyone can now get the stereoscopic effect.”

Use requires considerable concentration. A good deal of practice would be necessary to use the device in the situations required by military deployment. I tried out one of his rangefinders looking out from the back of The Observatory and, with effort and coaching, was able to accurately determine the distance to Cluniemore (house) on the opposite bank of the loch 1,093 yards away (1.01k). Checking with an OS map showed that to be remarkably accurate.

Amazingly, more than a century after the device was manufactured, it is an accuracy that is consistent with that achieved by Forbes himself in tests undertaken in South Africa.

Forbes’ accuracy tests, from the journal of the Royal Society, 22 February 1902

Forbes’ speculated that he might sell the rangefinders for as much as £80 each – a hefty sum of money, equivalent to about £6,200 in 2020 prices, or a year’s wage for a skilled man at the turn of the nineteenth century. The logic of Forbes’ pricing was intriguing.

Let us look now at the value of such an instrument. I have been assured that if my range-finder does all I claim for it, and if each company had one such instrument, then the effect of shooting would be doubled. One hundred men would then be doing the work, as fighting men, of two hundred, a gain, at £100 per annum, a soldier’s cost to the country in peace, of £10,000 per annum per company. Supposing, then, that the instrument were to cost one-third of the 54 inch-base instrument used in the navy, or £80 (and it is probably less), then for every £80 of capital spent, the army is receiving interest at the rate of £10,000 per annum.

Expectations among the military appear to have been for a unit price of about one third of that amount.

Intriguingly the model that I tested had been part of the collection held at Barr & Stroud’s headquarters in Glasgow’s Anniesland. A former employee removed it from the headquarters when the company moved in 1992. The model pictured – offered for auction in 2017 – has an identical provenance. 

Doubtless Barr & Stroud retained these so as to understand their competition – or even to incorporate Forbes’ ideas in their own products. Navel rangefinders became very big business for the Glasgow firm.

Alas both parties were to be disappointed in the competition to produce smaller devices. When the Rangefinder Committee made its recommendation, it was for Marindin’s design. Sambrook suggests that Marindin was preferred from the outset, possibly because he had pre-existing inside connections with members of the expert committee. Sambrook even suggests that it was widely accepted that Marindin’s design was the least effective of the three.

Nonetheless, after a bumpy journey between adoption and active service, it was the Marindin that went into general use in the British army from 1907 onwards. Whatever its effectiveness, it was certainly much larger than Forbes’ design, and more than twice as heavy.

The Marindin rangefinder in use – from the device’s manual

The extant ‘Forbes’ rangefinders are probably all prototypes or demonstration models. Despite his disappointment, he was subsequently employed by the Navy, for whom he developed a further navel rangefinder, and a means by which submarines could communicate using converging means of light. Both went into service use.

Sambrook argues that the War Office’s entire approach to this procurement was foolish. Barr & Stroud had, by 1900, become genuine players in the precision optics market, despite often struggling to recruit sufficiently skilled workers in Glasgow. A strategic approach by the War Office and the Admiralty would have been to work more closely with this firm, fostering its growth with government contracts. Instead, the government hoped to encourage competition, but, in this thesis, at the cost of nurturing a real giant of precision optics.

Had this been the British approach, it is far more likely that Forbes would have felt compelled to work with the Glasgow firm, rather than the German Leitz. Barr & Stroud did endure for most of the twentieth century, although it lost its independence in 1977 and today, save for its brand being used, has essentially disappeared.

Optical rangefinders have since been superseded by devices that utilise lasers, radar and sonar. Leica, for example, make a device similar in scale to Forbes’, which retails in 2020 for £2,900.

There is a fascinating Phd thesis on the history of optical munitions in the UK during this period here.

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