LECTURE III. 77 



In Lucae's instrument the perpendicular ray is given by the 

 intersection of two crossed threads^ the point of the pen having 

 to be placed in the same vertical line as the point of the object 

 to be dehneated. In Wirsing's instrument the point of the 

 pen is fixed perpendicularly under the minute aperture of the 

 diopter. The glass plate being adjusted in the horizontal 

 plane^ and the skull so placed that the plane upon which the 

 delineation is to be projected, runs parallel with the glass plate, 

 we draw the lines and points upon the glass plate, by moving 

 the instrument thereon in the requisite directions. 



I possess Lucae's instrument, and having practised with it 

 for some time, must say that one may, in a comparatively short 

 time, obtain by it a correct outline, which is, however, some- 

 what coarse, as the glass plate but imperfectly takes up the 

 liquid whether it be common or lithographic ink. In the prac- 

 tical use of this instrument it is especially requisite to pay 

 attention to the distribution of the light. Whilst for other 

 drawing purposes a side light is preferred, and studios have 

 only one large window for the proper distribution of light and 

 shadow, geometrical drawings, on the other hand, should be 

 taken in a glass house, where all is light, and there is no 

 shadow. The fine aperture of the diopter, too, through which 

 we look, cuts ofi" so much light, that when the light comes in 

 from one side, the black threads, or the point to be drawn 

 on the darker side of the object is invisible, and we thus have 

 an imperfect drawing. To obviate this, I have often artificially 

 'illuminated the dark side "by means of a candle or a lamp ; but 

 this is only a poor expedient, and becomes further objection- 

 able on account of the glass plate often becoming heated. 

 A mirror conveniently arranged will, as Lucae rightly observes, 

 serve the same purpose. Though geometrical drawing, so 

 long as it is given in natural size, admits of some measure- 

 ments being taken as easily as on the object itself, it cannot 

 be denied that, to common observation, it presents an appa- 

 rently incorrect image, and that our common mode of viewing 

 objects corresponds more to the perspective than the geo- 

 metrical. Strictly speaking, it corresponds to neither, and 

 stei'eoscopic views alone can render the image of the skull as 



