212 American Quarterly Microscopical Journal. 



to be measured is held in position upon the moving plate by 

 the clips shown in the cut. Instead of two parallel springs 

 there is a single cord attached to the center of the moving 

 slide, which runs on the guide pulley (</), and is attached to a 

 spring, not shown in the cut. which is fastened to a pin on 

 the back-side of the bed, a little to the right of and below b. 

 The action of the spring, therefore, is wholly in the line of 

 the screw, and as the direction of the cord falls a little below 

 the motion of the slide, it has a slight tendency to keep the 

 slide in contact with its seat without introducing friction. 

 The screw {c) moves the whole bed-plate, including the pre- 

 cision screw {b). The whole comparator has a circular move- 

 ment in the socket (/), attached to the original sub-stage {e) 

 of the microscope. The Beck filar micrometer is shown at 

 h, and an eye-piece with a micrometer, having some advantages 

 over the usual form, is shown at /. Slow motion to the tube 

 is given through the lever g. 



The operation of using the comparator is as follows : 



After the slide containing the graduations to be compared 

 has been placed in proper position under the objective, with 

 the right hand, the screw-head {b) is set at the zero of position; 

 with the left hand, line i is brought in contact with a single 

 line of the eye-piece micrometer; w^ith screw b, line 2 is 

 brought in contact with the fixed line of the eye-piece microm- 

 eter, and the number of revolutions and parts of a revolution 

 are read off. Screw b is then brought back to zero, and the 

 setting is made on line 2 by means of the screw c. In moving 

 over the space from line 2 to line 3, with the screw b, it will be 

 seen that ih€ same part of the screu< is used as in going from line 

 I to line 2. Hence, the comparison of these two spaces is 

 independent of the errors of the comparing scren'. 



The number of spaces which can be compared in this way 

 is only limited b}' the length of the screw c, and the length 

 of the opening through the bed-plate. 



Again, suppose we measure the spaces i, 2, 3, 4, . . 100, b)' a 

 continuous forward motion of the screw. Such measures will 

 involve all the errors of the screw itself. But if after the 

 measures are made, we set the screw-/^ back at zero, turn the 

 ruled plate around 180', and set on line ico with screw c, the 

 continuous forward motion of the screw b from line 100 to line 

 I will be over the same part of the screw as from line i to line 

 100. In the first case, the screw measures the accumulated 



