Outline of the horse
A thin, warm orange is visible around the contours of the horse. This is most probably the 'laying-in' or first paint layer, accurately marking out the position and outline of the horse.
Contour of the horse's hind leg
It seems possible that Stubbs worked up the figure of the horse to near completion at this point, adding only final touches after the landscape had been completed. The landscape was applied up to this outline, occasionally overlapping it but more usually following the contour carefully. At points where the two layers do not meet exactly, and it seems that this was deliberate, the orange underlayer provides a warm, soft edge that vibrates against the strong greens of the middle distance, as shown in the detail below. In this way, the sharply observed horse is not defined by hard edges but rather by the relationship of its pure, clear colour to the surrounding background.
Detail of foreleg and landscape
The x-radiograph provides little additional information. The thickness and density of the ground layer dominate the image and the thin, dark paint layers of the horse hardly register on the plate. However, it is clear that there are few if any alterations to the composition and the dark edges along some of the contours in the x-ray illustrate those points where paint layers do not quite meet and the thin orange outline remains exposed/uncovered - for example along the horse's forelegs.
X-ray detail of the lower left corner. The rear hoof is just visible at the centre.