Capt. Kater's account, &c. 
317 
Two small rectangular pieces of plate brass were prepared, 
of the same size, and about the tenth of an inch thick ; one 
of the surfaces and one side of each were ground perfectly 
flat, and the surfaces being placed in contact, two fine dots 
were made on the plane sides, close to the edges, as in — 
the accompanying figure. These pieces were intended • • 
to be applied to the extremities of the object to be 
measured, the dots serving as points, the distance between 
which was to be ascertained. 
In order to keep the brass pieces in their proper position, 
and at the same time to ensure, in every case, an equal pres- 
sure, two springs were made to slide along a mahogany 
rule, divided into inches. These springs projected nearly 
at right angles from the rule, and being set at the required 
distance from each other, retained by their pressure the brass 
pieces steadily in the situation in which they were placed. 
The micrometer microscope used on this occasion, differed 
essentially from that which is commonly employed. The 
microscope itself was carried along by the micrometer screw, 
instead of the motion being confined to its cross wires. 
By this construction, which was suggested by Dr. Young, 
no error could arise from the image not being in the same 
plane with the wires ; and it gave me, besides, the advantage 
of applying an object-glass, of whatever power I pleased, to 
the microscope, without altering the value of the divisions of 
the micrometer. 
Having placed the rectangular pieces of brass, with their 
surfaces in contact, and the sides on which the dots were 
made in the same plane, they were confined in this position 
between the springs before described. 
The following observations were then made with the 
