168 



MEMOIKS OF THE NATIONAL ACADEMY OF SCIENCES. 



To ascertain if the pin hole is truly vertical to the apex of the style, we take sight through 

 the former over the latter on some point ot the object to be depicted under the glass, and wheel 

 the instrument around the point acting as the center 180 degrees. If the pin hole is vertical, 

 the apex of the style will still cover the point on the object; if it is not vertical, we loosen the 

 binding screw (/, Fig. 24) and adjust the arm (e). 



The frame of our apparatus is 35 cra long, 28 cm broad, and 43 cm high ; but some approximate 

 size will do as well. The cross pieces which secure the upright supports are not placed at the 

 top of the frame, but some 12 C1 &quot; from it. There are two reasons for this: first, that no shadow 

 may fall on the skull to obscure the vision of the operator, and second, that a horizontal surface 

 may be afforded to support the mounted needle. The plane of the cross pieces must be perfectly 

 parallel with that of the plate of glass. 



FIG. 23. Apparatus for orthogonal tracings. 



Flo. 24. The periglyph. 



The mounted ueedle (Fig. 23, d) alone is used when the datum plane lies horizontally, as in 

 outlining the vertex, side, and base of the skull ; but when the plane stands vertically, as in tracing 

 the anterior and posterior views, the square is also employed to secure the desired adjustment. 



For facilitating the adjustment of the skull accurately and readily in any position, and for 

 maintaining it in position, we have found nothing to excel the sand box. The most elaborate 

 mechanical contrivance could not, we imagine, answer the purpose better. 



In this series, furthermore, the skulls were so fragile that they did not admit of the applica 

 tion of any crauiphore that would produce the least pressure. 



The following is the method of operation : Place the skull on the bed of sand, pressing it down 

 until it stands firmly. By means of the lever raise the sand box until the skull is nearly or quite 

 on a level with the slots in which the glass is to fit. Orient the skull in the sand with the aid of 

 the mounted needle, or the square, as the case may require; put the varnished glass in place; by 

 means of the periglyph make the desired tracing; take off the glass. If a positive picture is 

 desired, trace over the scratched drawing on the reversed side of the glass with ink. When the 

 ink is dry, proceed to make the imprint. Lay unglazed paper on the inked figure and press it 

 firmly down with one hand to prevent slipping; raise a small portion of the paper with the other 

 hand; breathe in one spot upon the ink sufficiently to moisten it; replace the paper and rub it 



