In the diagram, B is the Lateral background; consisting of a shed 37 metres or about 120 feet, long, the front of which is open, and divided by vertical and horizontal threads into spaces 5 centimetres, or about 2 inches, square, and by broader threads into larger spaces 50 centimetres, or about 19¾ inches, square.
At C and C, 37 metres, or about 120 feet, apart are "fixed" backgrounds, with vertical threads 5 centimetres, or about two inches, from their centres, with broader threads 30 centimetres, or about 12 inches, from their centres.
For some investigations, readily distinguishable in the plates, "portable" backgrounds are used, consisting of frames 3 metres wide by 4 metres high,—about 10 feet by 13 feet 4 inches,—over some of which black cloth and over others white cloth is stretched, all being divided by vertical and horizontal lines into square spaces of the same description as those of the lateral background.
These portable backgrounds are used when photographing birds and horses, and also wild animals when possible to do so.
L. A lateral battery of 24 automatic electro-photographic cameras, arranged parallel with the line of progressive motion, and usually placed therefrom about 15 metres or 49 feet.
Slow movements are usually photographed with lenses of 3 inches diameter and 15 inches equivalent focus; the centres of the lenses being 15 centimetres, or about 6 inches, apart.
Rapid movements are usually photographed with a portable battery of cameras and smaller lenses.
The centre, between lenses 6 and 7, is opposite the centre of the track T.
For illustrations comprising both "Laterals" and "Foreshortenings," cameras 1 to 12 only are used.
When "Laterals" alone are required, cameras 13 to 24 are connected with the system and used in their regular sequence.