Subsequently the authorities and their committee turned to Mr. (now Sir William) Richmond, R.A., whose veneration for St. Paul's dates from childhood. His interest in mosaic work caused him to study carefully the principles of design which obtained in Italy, Greece, and Asia Minor, during the best times of the Byzantine Empire.[104] Sir William has adopted the old plan of glass tesserae or cubes, and of four shapes--the cube, double cube, equilateral triangle, and a longer form with sharp points. They are of eight to ten tones of colour, and are put into position on the spot, being joined together by a mastic cement which resembles that used by Andrea Tafi in restoring the mosaics in the Baptistery at Florence. This cement in time becomes quite hard. The cubes with their complex facets are not joined close together, but separated by one-sixteenth to one-fourth of an inch, the better to reflect the light, so as to give a rich and soft texture. They are made at Messrs. Powell's workshops. Sir William has done a great deal more than design. He has, so far as this country is concerned, caused us to acquire a new art, while he has restored an old one. The workmen, who are all natives, have been trained by him. Accustomed only to the smooth, pictorial mosaics of thin plates of glass put together in the workshop, he had to teach the Messrs. Powell and their staff both how to make the glass cubes, and how to put each one separately into its place in the cement on the wall or roof. As our cathedrals are sermons in stone, so these adornments are intended to be illustrated sermons in glass. Beginning with the Creation, and including those, Pagans as well as Israelites, who prepared the way and led up to the Fulness of the Time, we are here taught the leading features of that progressive truth which has been revealed. The difficulty in dealing with the lofty blank spaces of the dome will be not to go too high up, and not to come too far down. At the time of revising these lines (August, 1899) the decoration of this part of the cathedral has advanced no further than the quarter domes of those alternate arches which tested so severely the genius of the Surveyor. In the four, taken as a whole, the general subject illustrated will be St. Paul's Gospel of the Resurrection from the early verses of 1 Corinthians, XV.