The spring of 1850 saw him once again settled in Dublin, with a great accumulation of work on hand. Part of the summer was spent in collecting Algae on the coast of Antrim; and he met again his friends Asa Gray and his wife, who were visiting Europe. Another acquaintance made at this time, which ripened into a warm friendship, was the result of the finding by Mrs Alfred Gatty, well-known as a writer of fiction, of the _Chrysymenia orcadensis_ of Harvey at Filey, in fruit for the first time--the examination of which convinced Harvey that the Orkney plant was only a variety of _Chrysymenia rosea_ (_Lomentaria rosea_ Thuret). Mrs Gatty became a useful ally in the collecting of seaweeds, and a valued friend; Harvey's influence is seen in her _British Seaweeds_, published in 1863. The year 1851 saw the completion of the _Phycologia Britannica_, and he at once set to work on his _Nereis Boreali-Americana_, published in three parts in the _Smithsonian Contributions to Knowledge_--a work of 550 quarto pages containing an account of all the known species of North American Algae, and 50 coloured plates, lithographed as usual with his own hand--a fine piece of work, and one which has not yet been superseded. This was a time of strenuous labour, for already he was planning a still more extended foreign tour; but he found time in the autumn of 1852 for a trip to Switzerland with Sir William Hooker and other friends. In August, 1853, Harvey set out on the most extended scientific expedition of his life. So far his collecting had been done in Europe, South Africa, and North America. Now he was to visit the Indian Ocean and Australasia, and to investigate their seaweed flora, as yet but little known. A short stay was made in Egypt, and a sea-shore ramble at Aden yielded _Padina pavonia_ and a few other seaweeds, but otherwise he made no stop till Ceylon was reached. There he travelled a good deal, but seaweed collecting was not so successful as he had hoped. Some of the places explored proved unproductive, and the prevalence of the monsoon rendered collecting difficult or impossible. But the last three weeks, spent at Belligam Bay and Point de Galle, yielded excellent results, and he proceeded to Singapore en route for Albany, with a collection of about 5000 specimens of Algae.