See manuscript’s transcription
Overview of Auxilia Calculi ex Ductu Linearum (1680 – 1716)
This text doesn’t concern directly perspective, but it shows a very important rule used in it, as Leibniz himself writes at the end of the manuscript. It deals with the properties of similar right triangles, one of which is generated by drawing a segment between a cathetus and the hypotenuse, perpendicular to this cathetus. The common vertex of the two triangles will be, in its application in perspective, the eye of the spectator, as it will be clear in the first part of Scientia perspectiva.