INTRODUCTION 



25 



the frame is simpler, and consists of a felt-covered metallic foot, of oval 

 form, which bears as the upright, a graduated steel rod. The curved 



FIG. 15. The perigraph of Lissauer, really a diagraph, differing but a little from that of 



Martin. (After Wetzel.) 



needle may be rotated so that the curve lies at any plane without chang- 

 ing the exact position of the needle point above the pencil. 



FIG. 16. Broca's stereograph. (After Topinard.) 



Stereograph of Broca. In the stereograph of Broca (Fig. 16) , we meet 

 with another principle, that of a swinging stand that carries the needle 



