2288 



Journal of Applied Microscopy 



contact, an electric circuit is completed through a battery, b, a resistance, r, 

 a telephone receiver, te, a nut, //, and thence through the screw, sc, and stage, s, 

 to the battery. At each make or break of the circuit at the point, /, the tele- 

 phone speaks. To use this apparatus, move the stage by rack and pinion, or 



Fig. 2. 



other adjustment, till the desired object is on the cross wire, bring the screw, sc, 

 into contact with the stage, 5, when the telephone speaks observe the reading, 

 R^, of the disc, d. Get the other side of the object coincident with the cross 

 wire, bring the screw into contact again and get a new reading, R, of disc. 



Fig. 



Diameter of the object will be Rj-R, Fig. 2 shows the micrometer alone, and 

 Fig. 3 the whole apparatus for microscopic measurement. This method has 

 been tested exhaustively for two years and is quite reliable if two conditions are 

 observed. (1) Both the contact surfaces of the screw and the stage must be 



