A novel telescope, developed for military use in 1857, was highlighted by The Engineer as having some interesting civil applications. Invented by M Porro, the Napoleon III eye-glass was designed to help determine the range of artillery and spy on distant objects.
We can imagine a stumpy little man in a crowd having one fitted into the handle of his umbrella, elevating the same above the shoulders of the tallest man who could by any possibility stand in his way.
’Notwithstanding the high price at which this article is sold (about £6 each), the demand for them, which still continues, affords the best indication of their practical utility,’ said The Engineer. ’At a distance of 1,000 metres [it can see] a space of from 45 to 50 metres, corresponding to about 75 men arranged in line.’
According to Porro, ordinary long-distance glasses were ineffective in military situations as they took too long to set up and adjust. To combat this, Porro developed a telescope small enough to be concealed by the hand and which, he claimed, would be much quicker to use.
’The length in the direction of the visual rays is about 36 millimetres, or 1.5 inches, and it is mounted on a convenient handle,’ said the article. ’A little disc with a milled edge, on which the thumb rests, allows the focus to be taken without having recourse to the method of drawing out.’
As well as its small size, the device could be converted into a polemoscope – a field glass that can see objects that aren’t directly in front of the eye. As The Engineer noted, this could also be useful in civilian life. ’We can imagine a stumpy little man in a crowd having one fitted into the handle of his umbrella, elevating the same above the shoulders of the tallest man who could by any possibility stand in his way.’