Fig. 5. represents the section of a solid prismatic piece of glass, within which both the reflections requisite arc effected at the surfaces a b, b c, in such a manner that the ray f g, after being reflected first at g, and again at h, arrives at the eye in a direction h c at right angles to g.
There is another circumstance in this construction ne cessary to be attended to, and which remains to be ex plained. Where the reflection was produced by a piece of plain glass, it is obvious that any objects behind the glass (if sufficiently illuminated) might be seen through the glass as well as the reflected image. But when the prismatic reflector is employed, since no light can be transmitted directly through it, the eve must be so placed that only a part of its pupil may be intercepted by the edge of the prism,, as at e, Fig. 5. The distant objects will then be seen by this portion of the eye, while the paper and pencil are seen past the edge of the prism by the remainder of the pupil.
In order to avoid the inconvenience that might arise from an unintentional motion of the cyc, the relative quantities of light to be received from the object, and from the paper, are regulated by a small hole in a piece of brass, which by moving on a centre at c, Fig. 6. is capa ble of adjustment to every inequality of light that is like ly to occur.
Since the size of the whole instrument, from being so near the eye, does not require to be large, I have, on many accounts, preferred the smallest size that could be executed with correctness, and have had it constructed on such a scale, that the lenses are only 4ths of an inch in diameter.
Though the original design and principal use of this instrument is to facilitate the delineation of objects in true perspective, yet this is by no means the sole purpose to which it is adapted; for the same arrangement of reflec tors may be employed with equal advantage for copy ing what has been already drawn, and may thus as sist a learner in acquiring at least a correct outline of any subject.
For this purpose, the drawing to be copied should be placed as nearly as may be at the same distance before the instrument that the paper is beneath the eye-hole ; for in that case the size will be the same, and no lens will be necessary, either to the object or to the pencil.
By a proper use of the same instrument, every pur pose of the pentagraph may also be answered, as a paint ing may be reduced in any proportion required, by placing it at a distance in due proportion greater than that of the paper from the instrument. In this case a lens becomes requisite for enabling the to sec at two unequal distances with equal distinctness; and in order that one lens may suit for all these purposes, them is an advantage in carrying the height of the stand ac cording to the proportion in which the reduction is to be effected.
The principles on which the height of the stem is ad justed will be readily understood by those who are accus tomed to optical considerations. For, as in taking a per spective view, the rays from the paper are rendered pa rallel, by placing a lens at the distance of its principal focus from the paper, because the rays received from the distant objects are parallel ; so also when the object seen by reflection is at so short a distance that the rays receiv ed from it arc, in a certain degree, divergent, the rays from the paper should be made to have the same degree of divergency, hi order that the paper may he seen dis tinctly by the same eye ; and for this purpose, the lens must be placed at a distance less than its principal focus. The stem of the instrument is accordingly marked at certain distances, to which the conjugate foci arc in the several proportions of 2, 3, 4., Stc. to I, so that distinct vision may be obtained in all cases, by placing the paint ing proportionally more distant.
By transposing the convex lens to the front of the in strument, and reversing the proportional distances, the artist might also enlarge his smaller sketches with every desirable degree of correctness, and the naturalist might delineate minute objects in any degree magnified.
Since the primary intention of this instrument is d•eady, in sonic measure, answered by the camera ob seura, a comparison will naturally be made between t hem.