[For Part 1, of which this is a continuation, see https://villacastagnadaylesford.com.au/2018/11/12/views-of-the-colosseum-from-the-north-1-luigi-rossinis-panorama/
Gaspar van Wittel was unusual, if not unique, in using a camera obscura set up on site. The gridded image on the camera obscura screen was carefully transposed to large drawings, many of which survive. It is likely something of the kind occurred when van Wittel first created his Colosseum composition, exemplified by a version with Robilant + Voena in 2008 (Fig. 1). This is datable to 1707. His first version probably dates to 1703.
Van Wittel’s viewpoint in plan. (Left-right orientations adopt the point of view of the painting.) Looking at the van Wittel Colosseum (Fig. 2) starting at the point where the upper arcades have collapsed at the right, there are 13 arches visible to the left of this point before they curve out of sight (Fig. 3). Starting at the same point, one can see straight down the first and second arches. A line through the middle of the pier between these gives us one line running through the viewpoint. A tangent drawn to the pier at the left of the thirteenth arch provides another. These intersect at point A1, in front of building G, which ought to be where van Wittel placed his camera obscura.
Other sight lines can be plotted on Nolli (Figs 4, 5.) From the line of the break in the upper outer bays, there are 3 bays of the middle row of arches, as Nolli confirms, followed by 8 bays of the inner row of arches before the Colosseum curves around at the right.
The start of the apse of SS. Giovanni e Paolo lines up with the right edge of a house and a little to the left of the Colosseum.
The right edge of the Arch of Constantine lines up with a point near the broken aqueduct near SS. Giovanni e Paolo about two arches width to its left.
The centre of the Meta Sudans lines up with the right side of the central arch of the Arch of Constantine.
A point to the right of the transept of S. Giovanni in Laterano lines up with the edge of the Colosseum.
These all converge near point A1, but slightly to the top left (north west).
Averaging all this data, one can conclude that van Wittel’s painting, with its broad extent, is taken from near point AF.
Van Wittel’s viewpoint in elevation. Van Wittel’s horizon is aligned with the middle row of arches of the Colosseum, the bottom of the attic of the Arch of Constantine, and above the top of building F. As in modern photos and Rossini’s Esquiline view show (Fig. 6 and Part 2, Fig. 3), the level of the top of the temple platform (just visible at the right) comes at the middle of the lower row of arches of the Colosseum. This means that van Wittel’s viewpoint is quite elevated. Taking into account the plan viewpoints, the only plausible point for the point where he set up his camera obscura is on the upper storey of building G. In Rossini’s panorama view (Part 1, Fig. 8) this is shown to be quite high, higher than building F and comparable with the second level of Colosseum arcades. In Rossini’s Esquiline view the line of the middle of the second Colosseum arcade is aligned with the upper storey windows of building G.
It seems fairly certain, therefore, that van Wittel set up his camera obscura in one of the rooms in the upper level of building G.
Roads in van Wittel. Rossini reveals the various routes across the space to the north-west of the Colosseum, which can be indicated on Nolli (Fig. 7). The main routes were from the Via Labicana (and ultimately S. Giovanni in Laterano), throught the Arch of Constantine to Piazza S. Gregorio, through the rch of Titus to the Campo Vaccino and the city centre, and up the road to SS. Andrea Apostolo e Bernardino. There was a secondary road up Road 5. These routes worked there away around two mounds, mounds 1 and 2 (they had been levelled by the time of Rossini but their location is still evident there). A nodal point, point L, lay between them.
In van Wittel (Fig. 8) the foreground has probably been modified somewhat from an optical view. Mounds 1 and 2 are visible, but shifted somewhat to the left. The temple platform is visible, and the low space beside it that forms street 5. There are various fallen column drums in this hollow, and other pieces of masonry. These may be largely fictitious, as is customary in such foregrounds, based on the fallen column drums higher up on the platform at the right, and further up road 5.
 Christoph Lüthy, ‘Hockney’s secret knowledge, Vanvitelli’s camera obscura’, Early Science and Medicine, vol. 10, issue 2, 2005, pp. 315–339.
 Gaspar van Wittel , View of the Colosseum and the Arch of Constantine, c. 1707. Oil on canvas, 48.5 x 108 cm. With Robilant + Voena in 2008. Signed on the capital at the centre: GASPARO VAN WITEL ROMA. Purchased in Italy, together with the View of Venice, by Friederik Sigmond van Bylandt (1749-1828); by inheritance to Anne Visser, née van Bylandt (1866-1929), whose coat of arms was formerly visible on the back of its companion piece; London, Christie’s, 16 December 1998, lot 69. Exhibitions: London, Robilant+Voena, Vanvitelli, 18 November – 19 December 2008, no. 4. Literature: L. Laureati, ed. by, Vanvitelli, exhibition catalogue, Turin 2008, pp. 29-31; L. Laureati, entry in the dealer publication on this painting..’
 Laureati, 2008, in Robilant + Voena, 2008: ‘Gaspar van Wittel presumably painted this View of the Colosseum, (which is slightly smaller than the version in Holkham Hall, that is identical in format and dated 1716), in 1707, because its pendant (Venice. The Bacino di San Marco, looking towards the Doge’s Palace and the Piazzetta, with the Bucintoro and other shipping; London, Christie‟s, 16 December 1998, no. 70. Fig. 1) is dated that year.’
 Briganti, 2nd ed., has examples beginning in 1703 (London, Briganti 55), 1716 (Holkham, Briganti 56) then 57, 58, 59, 60, 61. All are oil on canvas. Robilant and Voena in 2008 published Robilant and Holkham versions (Holkham, no 33 pp. 26-28; Robilant and Voena, no. 4, pp. 29 to 31). The Robilant version dated 1707 on basis of dating its pendant, a view of Venice, both sold Christie’s, London, 16 December 1998, lot 69.
 In discussing the Robilant + Voena version of Van Wittel’s composition Laureati identifies the viewpoint as being located in the gardens of S. Francesca Romana behind (see Robilant and Voena catalogue).