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By Root 738 0

but was galled to discover that Alan had replaced him at a salary of £900 per annum. The remainder of his income came from private business, work for the Convention of Scottish Burghs and occasional articles in the learned journals. His expenditure was steady but burdensome. In addition to the cost of keeping a household, an office and a regular travelling itinerary, he was also devoted to the welfare of his apprentices. In part, he had arranged his life in order to allow himself freedom, but he had also deliberately chained himself to his duties and dependants. The more commitments he undertook, the more good works he piled up, the more firmly he was anchored to the bourgeois life. Security came from the immutable things in life – a family, a block of stone, a lighthouse, an annual salary. But all the fortune and stability in the world could not entirely silence his worries, and he ended up marking the same insecurities on his children.

The most significant of the relationships that Robert, and later Alan, forged with their engineering contemporaries was with a pair of Parisian brothers, Leonor and Augustin Fresnel. By the 1820s, the Fresnels were to optics what Smeaton was to seatowers. Their work was singular for several reasons. Firstly, they were amongst the few scientists in Europe at the time to take the study of light seriously, and secondly, they devoted much of their time to finding practical applications for their experiments. Most importantly, from the Stevensons’ point of view, the Fresnels were working closely on the perfect form of the lighthouse lens.

The link between the two families was first forged during Robert’s tour of the French lights in 1820. Robert had taken himself off to see Corduan, the most famous of Europe’s early lighthouses. The light had been built originally in the fifteenth century, and then repaired with much ceremony by the architect Louis de Foix in 1570 under orders from Henri III. Since the French King believed that all public works should reflect the greater glory of the monarchy, Corduan was reconstructed to look more like a classical temple than a lighthouse. Tiered like a wedding cake and decorated down to the last curlicue, it was the most elaborate lighthouse in the world, far exceeding even Winstanley’s subsequent flourishes on the Eddystone. It took twenty-five years to complete and drove its architect almost to madness. Unfortunately, it was only after it was completed that de Foix realised the light was more exposed to the sea than he had initially supposed. In despair, he composed a poem to be engraved on the side of the building, challenging the gods to hurl their worst at his architectural wonder and cursing them for their indifference to his trials. In 1612, the gods responded. A bolt of lightning struck the top of the lighthouse and destroyed it. Construction work was taken over by another architect, Chatillon, who strengthened de Foix’s foundations, while radical work in the 1780s swaddled the whole structure in a plain casing of fresh stone.

By the time that Robert saw it in the 1820s Corduan was more an example of archaeology than architecture, with so many succeeding layers of work outside and in that the original structure barely survived. It was less the building than the lighting that interested him, however. While Robert was still experimenting and refining his silvered reflector lamps in the Scottish lighthouses, Augustin Fresnel had taken a different approach and had sought to magnify the beam by placing reflectors behind the light and prisms in front. The difference between catoptric lights (in which the lens or reflector was placed behind the flame) and dioptric (in which the lens was placed in front) was akin to the difference between candles and electric light. Even the first tentative versions of the lenses strengthened the beam from a maximum of 1,000 candlepower to around 3,000 candlepower. Admittedly, the first lenses were clunky, primitive objects, more like giant myopic spectacles than the elegant prisms of later years. But the leap had been