396 5. /. SHAND 



tiny squares is a most aggravating business, and the fewer the 

 squares, the greater the probable error in the result. On the whole, 

 the advantage seems to be with the Rosiwal method, as being both 

 simpler and more direct than the others. The practical disad- 

 vantages of the method are two: First, the making of very many 

 minute measurements by the aid of the scarcely visible scratches 

 on the eyepiece micrometer puts a severe strain upon the eyesight, 

 as well as upon the patience, of the observer. After an hour or 

 two of such work I have sometimes been nearly blind. Secondly, 

 the writing down and adding up of some hundreds or even thousands 

 oi measurements is itself a most tedious operation. 



To obviate these serious disadvantages of the Rosiwal method, 

 I have devised a stage micrometer which both makes the measure- 

 ments and performs the addition of them; it consequently effects a 

 great reduction in the time needed for the estimation, and inci- 

 dentally reduces the strain on the eyes to a minimum. The first 

 instrument was made from my drawings by Mr. T. A. Linton, at 

 the South African College, Cape Town, and 1 have pleasure in 

 expressing my appreciation of his excellent workmanship. 



The design is very simple, and will easily be followed with the 

 aid of the drawings (Figs, i, 2, 3). The rock section, mounted as 

 usual on a glass slide, fits into a rectangular brass sledge A, which 

 is movable to right or left of the observer, within another sledge B, 

 the movement being accomplished, and its amount recorded, by 

 the micrometer screw L. Sledge B moves in the same manner and 

 direction within sledge C, the movement being performed and 

 recorded by the micrometer screw R. Sledge C has no transverse 

 movement; it carries two runners on its under surface which 

 travel in grooves on the sides of the rectangular stage of the micro- 

 scope; the only movement of this sledge is to and from the observer 

 and is effected simply by hand. 



Suppose it is required to estimate the volume of augite in a 

 dolerite. The section is put in place and adjusted till one edge 

 of it appears against the point of intersection of the cross-wires 

 in the eyepiece of the microscope. The readings of screws R and 

 /. are written down. Then screw R is turned continuously until 

 a grain of augite is brought up to the cross (i.e., the point of inter- 



