A RECORDING MICROMETER FOR ROCK ANALYSIS 229 



Their work, as well as that of other students, indicates the need 

 of large numbers of measurements to insure results of requisite 

 accuracy, and the usefulness and popularity of the Rosiwal method 

 depends upon the simplification and standardization of the mechani- 

 cal and numerical operations involved. This was recognized in 

 1916 by Shand,^ who described a recording micrometer which 

 served both to make the measurements and to add the resulting 

 figures. This instrument, which can be attached to the stage of 

 any petrographic microscope, saves the large amount of labor and 

 eye strain previously involved in the adding of the data as well as in 

 the concurrent recording and observation. It has, however, one 

 important defect, as Shand himself recognized,^ in that it can be 

 used to measure but two constituents at a time (any one mineral 

 and the remainder of the 

 rock). In case more 

 minerals are to be esti- 

 mated, it is necessary to 

 repeat the operation. 



Having occasion re- 

 cently to make a number 

 of such estimates of rock 

 composition, and need- 

 ing some sort of device to 

 do the work, the writer 

 found it possible to over- 

 come the limitation of the 

 Shand micrometer by the mechanical device shown in Figure i . This 

 was constructed by Mr. Stanley Price, mechanician, of the State Uni- 

 versity of Iowa, according to the writer's specifications. It con- 

 sists essentially of a single sledge which is actuated by a series of 

 screws and nuts arranged in a string on a square rod at one side. 

 The screws are free to slide but are prevented from turning by the 

 square hole by which each is fitted to the square rod. The nuts 

 consist of the threaded sleeve and a graduated dial by which their 

 motion is recorded. The position of the sledge, which is held against 



^ S. J. Shand, "A Recording Micrometer for Geometrical Rock Analysis," Jour. 

 Geol., Vol. XXIV (1916), pp. 394-404. 2 Ibid., p. 399. 



Fig. I. — Improved recording micrometer. A is 

 a square rod, 5 is a screw with an axial square hole 

 which fits A, C is a nut and dial fitting the threads 

 oi B, Z) is the housing of a tension spring which pulls 

 the slide carriage toward the right. Parts slightly 

 separated to show relations oi A, B, and C. Nor- 

 mally the spring keeps these together. 



