The three-colour process consists of half-tone blocks printed in colours one over the other, but although they look well they are not particularly true to their originals. The reason of this is that each block is a little wrongly inked, as the tint of the pigment put upon it depends entirely upon the printer. He has of course a carefully coloured key given him to match for each block, but he never quite succeeds in doing more than get near it. There are line keys and colour blocks, and half-tone and colour blocks, and many other varieties of combinations of processes. The half-tone process is certainly responsible for much charming and valuable work, but it has done one very great harm not only to itself but even to literature, it has been the chief cause of the introduction of clay-laden paper (see Chap. III.). The beauty of photographic illustrations can be best seen in some of the recent French illustrated books in colour published by the Société des Amis des Livres. Other exquisite illustrations are to be found in Octave Uzanne’s books, many of them from the drawings of Paul Avril. The way in which many of these illustrations are made to show over the printed page is often quite charming. BOOKS TO CONSULT. BAER, L.--Die Illustrirten Historienbücher des 15 Jahrhunderts. _Strassburg_, 1903. BAYARD, E.--Illustrations et les Illustrateurs. _Paris_, 1898. BLACKBURN, H.--The Art of Illustration. _London_, 1896-1901. BONNET, G.--Manuel de Phototypie. _Paris_, 1889. BONNET, G.--Manuel d’Héliogravure et de Photogravure en relief. _Paris_, 1890. BOUCHOT, H.--Le Livre, L’Illustration. _Paris_, 1886. BOUCHOT, H.--Les Livres a vignettes. _Paris._ BRIVOIS, J.--Bibliographie des livres a gravures sur bois du XIX^e siècle. _Paris_, 1883. BROUGH, W. S.--Book Illustration. _Leek_, 1891. BULLOCK, J. M.--Art of Extra Illustration. 1903. CRANE, W.--Of the Decorative Illustration of Books. _London_, 1901. DAVENPORT, CYRIL.--Mezzotints. _London_, 1904. DOBSON, A.--(Chapter on Illustrated Books in Lang’s “The Library.”) _London_, 1881. DOBSON, H. A.--Bewick and his Pupils. _London_, 1884. DUCHOCHOIS, P. C.--Photographic Reproduction Processes. _London_, 1892. FARQUHAR, H. D.--Grammar of Photo-engraving. _London_, 1895. GERRING, C.--Notes on Book Illustration. _Nottingham_, 1898. GEYMET, T.--Traité de Gravure en demi-teint par l’invention du cliché photographique. _Paris_, 1888. GEYMET, T.--Traité de Gravure et impression sur zinc. _Paris_, 1887. GEYMET, T.--Traité de Photogravure. _Paris_, 1886. GEYMET, T.--Traité de Photo-lithographie. _Paris_, 1888. GROLIER CLUB, New York.--Catalogue of Engraved Titles and Frontispieces published in England during the sixteenth and seventeenth centuries. _New York_, 1898. HARDIE, M.--English Coloured Books. _London_, 1906. HENRICH, M.--Iconographie de las ediciones del Quijote de M. de Cervantes Saavedra. _Barcelona_, 1905. HINTON, A. H.--Handbook of Illustration. _London_, 1895-1905.