^{page 285 note *} I should here most thankfully acknowledge, seeing that the work was thereby so greatly facilitated, the extremely liberal and generous conduct of the Pacific Steam Navigation Co. of Liverpool; who, four times over, kindly and safely conveyed all the large packages of scientific instruments, free of expense, in one or another of their several magnificent steam-ships of 4000 tons burthen. These fine vessels start every month on their grandly oceanic voyages to South America via the Straits of Magellaen, taking Lisbon in their way; and form an almost luxurious, at the same time that they are both a speedy and yet admirably economical, method of passing and repassing between cloudy Britain, and its favourite little, historic Ally in the clear and sunny South.

Among those to whom my thanks are more particularly due, I trust to be excused for mentioning Captain Hamilton of the Aconcagua, Captain Graves of the Cotopaxi, Chief Officer Friend of the Liguria, Captain Hayes of the Valparaiso; and though last, by no means least, Mr Sanderson, the courteous Secretary of the Company, and Reginald Harrison, Esq., the active and ever watchful Medical Officer of Liverpool's most extensive and busy Port.

^{page 285 note †} Though the apparatus was put together at home, the several important parts of it were furnished by, and are altogether due to, the professional skill of M. Salleron, 24 Rue Pavèe au Marais, Paris ; and Mr Adam Hilger, 192 Tottenham Court Road, London.

^{page 288 note *} This was in fact adapting each part to our Wave-number Scale; for, without such reduction, a prism formed spectrum is nearly sixteen times longer in the violet, than it is in the red, region, as compared with a diffraction spectrum.