SECT. 3] 



APPLICATIONS OF THE GYROPENDTXLUM 



341 



MICROSCOPE 



OK*"" 



PENTA HORIZON SCREEN 



PRISM 



Fig. 13. (a) Arrangement of parts in the dipmeter shown in Fig. 13b. (b) The 5-in. dip- 

 meter for land stations, shown diagrammatically in Fig. 13a, with the horizon shutter 

 open and the mercury dish exposed. The resolving power of the optical train is about 

 1 second of arc. 



