234 J. MATTHEWS ON A NEW AND 



that which I show you. In this the points are so curved in opposite 

 directions, as to form two semi-circles, which may either be turned 

 quite out of the field, or be placed with their jDoints in apposi- 

 tion, so as to form a curve like the letter S, being capable, of 

 course, of assuming any intermediate position, so as, apparently, to 

 include an object between their points. They form, in fact, a 

 pair of callipers, and their uses are exactly the same. An 

 object having been placed on the stage, the points are advanced 

 on either side of it until they are in exact seeming contact 

 with its edges. If it be irregular in shape, or if the long diameter 

 do not correspond with the position of the pointers, the eye-piece 

 must be rotated until they attain the desired position. The object 

 is then removed, and the stage micrometer substituted, when the 

 dimension may be directly read off. This operation is exactly re- 

 peated for as many measurements in all directions as may be re- 

 quired, the eye-piece being rotated after each such measurement 

 as does not leave the pointers in a line with the divisions of the 

 scale. If it be required to count stride or markings of any kind, 

 then the points are to be set at some definite distance apart by the 

 stage micrometer. The object is then to be substituted for it, and 

 the counting effected. I cannot conceive a single case to which 

 this method is not easily applicable. Its cost is not great, for the 

 pair of points may be applied to any eye-piece for a price not ex- 

 ceeding ten shillings, and its ordinary use is in no way impaired 

 thereby. If greater delicacy be required, the points may be moved 

 by tangent screws, but these would seriously increase the cost, 

 and are really scarcely necessary. Indeed, an economy is the re- 

 sult of this instrument, since an eye-piece micrometer would cost at 

 least double the above amount, and must, after all, be referred to a 

 stage micrometer. I had at first some diflSculty in getting points 

 sufficiently fine, but Mr. Hislop has suggested to me the employment 

 of fine pendulum spring wire, pointed by a blow of a hammer in 

 one direction, and has very kindly assisted me by carrying his sug- 

 gestion into effect with great tact and delicacy. 



To sum up, I may say that the advantages which I claim for this 

 invention are these : — 



1st. Direct measurement, i.e., one without an eye-piece micro- 

 meter, requiring therefore no comparison or correction. 



