_Note._--Whilst on this subject, I may note that the painter in his large decorative work often had difficulties to contend with, which arose from the form of the building or the shape of the wall on which he had to place his frescoes. Painting on the ceiling was no easy task, and Michelangelo, in a humorous sonnet addressed to Giovanni da Pistoya, gives a burlesque portrait of himself while he was painting the Sistine Chapel:--
_"I'ho già fatto un gozzo in questo stento."_
Now have I such a goitre 'neath my chin That I am like to some Lombardic cat, My beard is in the air, my head i' my back, My chest like any harpy's, and my face Patched like a carpet by my dripping brush. Nor can I see, nor can I budge a step; My skin though loose in front is tight behind, And I am even as a Syrian bow. Alas! methinks a bent tube shoots not well; So give me now thine aid, my Giovanni.
At present that difficulty is got over by using large strong canvas, on which the picture can be painted in the studio and afterwards placed on the wall.
However, the other difficulty of form has to be got over also. A great portion of the ceiling of the Sistine Chapel, and notably the prophets and sibyls, are painted on a curved surface, in which case a similar method to that explained by Leonardo da Vinci has to be adopted.
In Chapter CCCI he shows us how to draw a figure twenty-four braccia high upon a wall twelve braccia high. (The braccia is 1 ft. 10-7/8 in.). He first draws the figure upright, then from the various points draws lines to a point _F_ on the floor of the building, marking their intersections on the profile of the wall somewhat in the manner we have indicated, which serve as guides in making the outline to be traced.
'Draw upon part of wall _MN_ half the figure you mean to represent, and the other half upon the cove above (_MR_).' Leonardo da Vinci's _Treatise on Painting_.]
[Illustration: Fig. 68. Interior by de Hoogh.]
To draw the interior of a cube we must suppose the side facing us to be removed or transparent. Indeed, in all our figures which represent solids we suppose that we can see through them, and in most cases we mark the hidden portions with dotted lines. So also with all those imaginary lines which conduct the eye to the various vanishing points, and which the old writers called 'occult'.