524 WRIGHT: PRECISION PROJECTION PLOT 



The device described below is similar in principle to that 

 adopted by Wiilfing, Johannsen, and Noll; but its design and 

 construction are different; it is built with special reference to 

 precision and convenience. A metal stand (fig. 1) supports an 

 electric lamp at C which illuminates a disk of frosted plate glass ; 

 this plate in turn supports an equatorial projection net, stereo- 

 graphic or angle (globular), printed on thick, transparent cellu- 

 loid. The two nets are 40 cm. in diameter and were reduced 

 photolithographically by the precision methods of the U. S. 

 Geological Survey, 14 the first from a carefully taken print of the 

 Sigsbee projection net, 15 the second from an accurate drawing 50 

 cm. in diameter of an angle (globular) projection net. On both 

 these nets, either one of which may be used, the curves are at 

 1° intervals and are sufficiently separated (nearly 2.5 mm. on 

 an average) that 0.1° can be read off without difficulty. The 

 celluloid disk rests on the glass disk and by means of center- 

 ing screws can be centered to the axis of rotation of the outer 

 steel ring which runs in an accurately turned bearing and carries 

 the tracing paper on which the measurements are plotted in the 

 positions indicated by the underlying projection net. 



Experience extending over several years with this apparatus 

 has shown that it meets the exacting requirements of accurate 

 work well and is, moreover, convenient to use. The tracing paper 

 is held in place by means of the hinged iron bars which pass over 

 the outside square ends of the rotating ring and clamp the paper 

 securely. 



Experience has also shown that the distortion of the stereo- 

 graphic projection has in certain instances an appreciable effect 

 on the attainable accuracy, and that for most purposes of calcu- 

 lation and of projection of optical data the angle (globular) 

 projection net is preferable. 



14 The writer is indebted to Dr. Geo. Otis Smith, Director of the U. S. Geo- 

 logical Survey for having had these reproductions made. 



15 Furnished to the writer through the courtesy of Geo. W. Littlehales of the 

 U. S. Hydrographic Office. 



