Subject

This may not be our Swaab but, from Google books and TIB, we see that in 1864 Trubner published a 56 page book by S. L. Swaab entitled 'Fibrous Substances, indigenous and exotic : Their nature, varieties, and treatment considered, with a view to render them further useful for textile...' and the TIB site is in German.

That's all we could find but then, 2020, our colleague Andrew Behan had a go and found more, as follows:
Samuel Leon Swaab was born on 4 January 1821 in Amsterdam, The Netherlands. He left The Netherlands, travelling from Rotterdam and arriving in the Port of London on 19 November 1849. He became a British naturalised subject and the 1861 census shows him and his wife, Antoinette Swaab (1820-1896), living at 9 Hunter Street, Brunswick Square, Bloomsbury with a female servant. His occupation was recorded as an Oculist (Own System). The Post Office London Directories of 1865, 1870 and 1875 record him as an Oculist still living at 9 Hunter Street and this is confirmed with electoral registers up to and including 1879.

The 1881 census shows him living at 10 Ampthill Square, Camden, with his wife and a female servant. His occupation remained recorded as an Oculist and electoral registers from 1882 to 1885 show him still at this address. He died, aged 65 years, on 6 January 1886. Probate records show that his address was 266 King Street, Hammersmith and that he died at this address. His will was proved with his widow being the sole executrix and his personal estate totalled £1,182-2s-10d. His widow continued to live at this address until her death on 12 February 1896 at which time her effects totalled £1,666-1s-7d. 

The catalogue for the 1862 International Exhibition has an entry in Class IV - Animal and Vegetable Substances used in Manufactures as follows: Swaab, S. L., Oculist, 9 Hunter Street, Brunswick Square. - Prepared India fibres, flax, hemp, and fibres converted in silk and cotton.

Oculist is another word for ophthalmologist or optician. Seems Swaab had two occupations.