950 EECORD OF CURRENT RESEARCHES RELATING TO 



attached to each plate, giving the form usually adopted. Instead of 

 two Microscopes, however, it is found better to use but one. The 

 Microscope plate is followed on the other side by plates terminating 

 in tempered steel tops which are at will either made free or clamped 

 firmly to the bed of the comparator. If one wishes to compare two 

 metres the method of proceeding is as follows : — 



(a) One stop is set at or near one end of the bed. 



(h) The metre with which comparison is to be made is placed in 

 position under the Microscope so that contact is made between the 

 end line and the zero line of the eye-piece micrometer. 



(c) The microscopic plate is then moved by means of the rack 

 and pinion till the other end line forms contact with the zero line of 

 the micrometer 



(d) The second stop is then brought up against the other end of 

 the plate and adjusted so that when contact takes place between the 

 stops, contact also takes place between the end line and the zero line 

 of the micrometei'. 



(e) Having made the adjustment of the stops perfect, the metre to 

 be compared is then placed in position. When contact is made with 

 the first stop by mechanical adjustment the end line is brought in 

 contact with the zero line of the micrometer. The Microscope plate 

 is then brought into contact with the second stop. If the other end 

 line is now in coincidence with the zero line of the micrometer the 

 two metres have the same length. By noting the number of divisions 

 which the end line falls short of or passes beyond the zero line of the 

 micrometer, the difference in the entire length can be found, the only 

 element yet unknown being the value of one division of the micro- 

 meter. 



After the comparison has been made it is better, as a matter of 

 precaution, to again compare the standard with the distance between 

 the stops. Since the stops can be set in actual contact with the 

 Microscope plate at either end it is obvious that this method admits 

 of a comparison of short spaces as well as of long ones. The only 

 criticism which it is imagined will be urged against this form of 

 construction is that founded on a doubt whether the contact between 

 the stops always indicates the same measured space. The arm of the 

 pinion has a head of about 2i inches in diameter. In his own case 

 the sense of touch has been so far cultivated that he is able to make 

 100 successive contacts without a single deviation exceeding • 000035 

 inch, and very few deviations reach -00001 inch. A comparator of 

 this form possesses one decided advantage over all others, viz. that 

 after the stops are once set any adjustment of the Microscope mai/ be 

 made ivilhout interfering ivith the comparison. The only condition 

 required is that the relation between the stops and the bed shall 

 remain unchanged during the short time required for the com- 

 parison. This does not usually take over ten minutes. 



In order to compare separate subdivisions of the same standard 

 we proceed as follows : — 



The stops are set e. g. equal to 1 decimetre. After the reading 

 of the first decimetre has been taken as indicated above, the bar is 



