CORE ORIENTATION 79 



chemist's. This film is placed exactly so as to receive the 

 image of the needle h and guide marks on the phosphorescent 

 disc c below it. The very luminous objective has an 

 aperture of /.3 and focal length of about 40 mm. and is 

 specially corrected for the refractive index of water; the 

 distance from the film is constant. In front of the objec- 

 tive is a small shutter plate h which opens only on pressure 

 being applied, on a rod projecting externally, when the 

 apparatus meets the core. The magnetic needle h is freely 

 suspended uncontrolled by any mechanism and is swung so 

 as to function even when the apparatus is tilted. Behind 

 it is the thin copper disc c covered with a substance insoluble 

 in water and containing calcium sulphide. (This is very 

 phosphorescent when properly made in the way employed 

 for this device.) It has the property of great luminous 

 emission. Black guide lines have been traced on the disc. 



A small distance from the above parts is a plate of phos- 

 phor bronze sufficiently thick and strong in which are four 

 holes of different diameters. These holes serve as guiding 

 points and enable one to ascertain whether the lead plate 

 has been displaced during the manipulation of the appara- 

 tus. Other guiding points enable the bronze plate to be 

 set; also all the rest of the movable parts of the device. 

 Against the plate is a lead plate for taking the core impres- 

 sion on its outside and the impression of the holes on its 

 upper face. 



For action the disc is taken out and made very phos- 

 phorescent by burning before the surface of the sulphide a few 

 centimeters of magnesium ribbon; this strongly excites the 

 phosphorescence so that the disc remains luminous enough 

 to enable one to read a watch in the dark for 4 or 5 hr. 

 This is then screwed back in and the lead plate put on and 

 the shutter closed. Now in a dark chamber the sensitive 

 film is fixed and the apparatus filled with water and closed 

 up, the water being as near as possible in temperature 

 to that in the borehole, avoiding air bubbles. This does 

 not affect the action of the apparatus at all. The instru- 

 ment is now ready to lower into the hole. 



