167
The few principles now laid down may suffice, in amultitude of cases, to enable the student to ascertain thecorrectness or incorrectness of the perspective of suchobjects as fall under his notice. It very often happensthat both architects and painters are not properly ac-quainted with the laws of perspective, and consequentlyapply them incorrectly. When geometrical knowledge ismore widely spread, many gross faults, which at presentoffend the discrimination of only a small number ofconnoisseurs, will be perceived by the public at large;and artists, finding more enlightened critics, will not beable to err with impunity. They will be compelled to studymore profoundly the applications of geometry to per-spective ; and thus, by enlightening the great mass of thepeople, though they may neither practise the arts, norpurchase the products of the artists, we secure for our coun-try more skilful artists and more admirable specimens oftheir skill. In the best days of Greece and Italy , it isprobable that almost every citizen could judge as cor-rectly as the artists themselves, of the magnificent workswhich adorned the temples and cities of these favouredcountries. When the public is an enlightened judge, ac-curacy of execution is a general talent. Works of artare then formed or constructed with that exactness ofproportion in all their parts, which is as indispensable toperfection in the fine arts, as in the structure of thosemachines which, though intended to perform some service,would not move at all, were not every part constructedwith geometrical precision.
Application of perspective to drawing the plans of ma-chines, and other products of industry. —When machines,or other works of art, are to be represented, it is some-times done in perspective. This method has an advantageover projection, inasmuch as it renders visible many ofthe parts which in that method conceal one another. Forexample, in representing the projection of a building byparallel lines, it is customary to make the vertical planeof projection either parallel to the front, or at right an-