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One end of a fine thread is fixed at @, passes through the 
rings #7, S, and is tightened by the hand so as to pass through 
the point P, where the ruler 7'Q meets the given Plan. The 
point p, where the portions of the thread QE, PS intersect, is 
the ‘perspective’ of P, or rather of P’ the point in the actual 
Plan to which P corresponds. 
That p is the ‘perspective’ of P, or the point of intersection 
of the straight line joining S and P made by a vertical plane at 
a distance HS from the eye, can be seen thus. To obtain the 
actual position of the eye, we must draw astraight line HS’ from 
towards us and perpendicular to the paper so that HS’=ES. 
S’ will then be the actual position of the eye. Again, the 
actual position of P (on the assumed scale) is obtained by 
drawing @P’ perpendicular to the paper and on the other side, 
so that QP'=VF. Now if we join S’P’, the point when this 
line meets the paper is the perspective of P’. This point 
manifestly lies in the intersection of the paper with a plane 
containing the parallels QP’, HS’, 7.e. it lies in the straight line 
EQ, and divides HQ in the ratio of HS' to QP, p fulfils these 
conditions, and is therefore the perspective of P’ required. 
By sliding the ruler, the perspective of every point in 
the plan, however complicated, may be arrived at; or, in other 
words, the section of any cone with any vertex and any base 
may be obtained. 
To determine the perspective of any point in an elevation, 
say m feet above P, draw a straight line through p parallel to 
the ruler, and where this cuts the thread from S through a point 
corresponding by scale (as marked off on the ruler) to n feet 
above P, is the required perspective of the point. 
There is a little difficulty, especially in some positions, in 
marking accurately the point p with a pencil however fine, 
owing to the effect of Parallax, as both threads cannot lie 
exactly in the plane of the Board, and also because they must 
be slightly pushed aside in order to mark with the pencil. 
