How to Do Mechanical Tricks / Containing Complete Instruction for Performing Over Sixty Ingenious Mechanical Tricks

by active 1894-1902 A. Anderson

Now it remains only to show the use of the instrument. Put yourself in front of the object you wish to represent. Put the frame in a perfectly horizontal position, slide the pane in it, and dispose the rule, E E’ ([Fig. 2]), in such a manner that, when looking through the little hole, O, you are able to see the object you want to draw.

Fig. 2.

Then, with a blue or other colored pencil, trace the outlines of the object on the glass coated with turpentine, the use of the latter being to allow the pencil marks to fix itself on the surface. One sees that the outlines thus obtained will be those of the real object as clearly as possible because they are traced as seen, so to speak.

But the principal object of the instrument is not so much exactness of outline as to get the exact proportion existing between the different sizes of the objects placed in different planes. We will try to show this last result by means of [another figure].

Suppose A B to be an object situated at a certain distance from the eye posted at o, the rays from the eye, O A O B, meets the instrument at a and b, and the image of this object is given by the line, a b.

Now, suppose A’ B’ to be another object situated beyond A B; the eye has not changed position, it cannot do so, with reference to the glass, on account of the small rule which is fixed; the image of the object A’ B’, will be a’ b’; thus, one has the true dimensions of A’ B’, in respect to A B.

It is precisely this proportion which must exist between the sizes of the objects placed in different planes, which constitute perspective. The instrument, therefore, well deserves its name of Perspectograph.

It will be observed that this apparatus obviates two difficulties: 1, that of the exactness of the sketch, in copying nature as it is presented to the eye; 2, that of perspective. Having the sketch on glass it is easy to transfer it on paper. Lift up the rule, E E’, so as not to be in the way, place oiled or transparent paper on the glass, and counter-draw the sketch on it.

You can then stick this paper on a cardboard, and, if the operator is a designer, he may reproduce in crayon a very fine drawing. For the shading he must use his own talent, the aim of the instrument not being to give a finished drawing, but only a sketch, vigorously exact, and in unexceptionable perspective.