Page:Journal of the Asiatic Society of Bengal Vol 7, Part 2.djvu/89

1838.] misplaced one-tenth of an inch out of the centre of the picture. As an example I have engraved two ruled images of a medal of Roaaa, belonging to Mr. LANG, C. S. with the deviation or distortion thrown in opposite directions. Few will believe that they represent the same object! In running down the relief (as in the cavity of the ear, and the front qf the forehead,) it will be seen that the engraved lines return and cover a part of the plate already engraved! There is to be sure an attempt to diminish the fault by lessening the deviation of the engraved lines :—thus, the one-tenth altitude may be made to give a devialion of only one-twentieth or one-thirtieth in the engraving (by lessening the angle of axis B—but the light and shade will be thus equally diminished, and the whole effect destroyed.

The mode in which Mr. BATE junior got rid of this difficulty in his patent instrument is then described—and it was its ingenuity which alone led me to send for one of the instruments to rule my Bactrian coins, rather than attempt to make one for myself, which I shall now be compelled to do.

My son, observing, that the thing to be desired was, a means of bringing the tracer down upon the medal, a quantity equal to the deviation of the etching-point was the straight line upon the plate; observing also that the process he was employing, transferred vertical sections of the medal to the plate,—proposed taking iisclined section. of the medal. A little consideration determined the seladtion of 4°, as being equidistant from the vertical and horizontal positions employed and this inclination completely fulfilled the purposes required. resw,mg the distortion altogether, and so far from Impoverishing the effect of light and shade, improving that effect, inasmuch as without diminishing its quantity it threw the light upon the representation of the medal at an angle of 450 to its plane, instead of as before in the direction of the plane of the medal’. The arrangement finally adopted is represented in fig. 2.

“The tracer c being now attached to the right-angled triangle efy and a friction roller substituted for itat *, the triangle (the motion of which was strictly confined to the plane of the diagonal a g,) moved d a quantity aLwaya equal to the distance of the tracer c from the perpendicular p. so that the etching-point described precisely the same line upon the plate 6 as the tracer described upon the surface of the medal a.”

Nothing could be more simple, efficient and correct than this provement, and though the merit of it has been contested by the French and by the Americans, I thought Mr. BATE justly entitled to his patent (of which by the way I have seen no specification yet in the Repertory) and willingly acceded to the terms he enjoined to my friends in Eng. land on consenting to make me one,—namely, that I should not make

B Is a second axis fixed on A at any convenient angle, carrying the arm which holds the diamond point or graver. •

This is not so comprehensible—the effect of light and shade depend. iserely upon the amount and direction of the deviation: and the smaller the relief of a medal, the more horizontally the light I. required to fail on It in order to exhibit parallel effects to those of more angular light on a high relief.