320 PROCEEDINGS OF THE AMERICAN ACADEMY 



(2.) The plates K and I are adjusted so that the defining lines of 

 the first standard to be compared shall fall under the microscopes M 

 and M^. 



(3.) The stops H and H\ having been brought into contact with 

 the plates K and I, are securely clamped. 



(4.) After the adjustments for position and for focus have been 

 made, successive contacts of the j^lates K and I are made with the 

 stops H and H^, and the microscopes M and M^ are read for each con- 

 tact. These operations having been repeated for each standard to be 

 compared, the values of (M^ — M) when reduced to a common unit 

 and compared inter se, will give the relations required. 



It is the experience of the writer that the microscope carriage can 

 be brought into actual contact with the stops, by means of the rack 

 and pinion movement, with greater certainty than it is possible to 

 make a coincidence of the micrometer thread with the defining lines 

 of the standard. The following test has been frequently tried, and 

 always with the most conclusive results. With a quarter-inch ob- 

 jective, in which the value of one division of the micrometer screw is 

 only 0.11 /x, a series of one hundred successive contacts with the stops 

 were made without disturbing the position of the micrometer thread. 

 The number of cases in which the deviation from the mean of the 

 first two or three readings of the micrometer was perceptible to the 

 eye were noted, and the amount of the deviation was estimated in 

 terms of the apparent width of the micrometer thread. One hundred 

 readings of the micrometer thread were then taken for coincidence 

 with the defining line of the standard, the plate K remaining station- 

 ary. A comjjarison of the results obtained in this way has always 

 been found to be in favor of the stop contacts. "With a little experi- 

 ence on the part of the observer, the stop method admits of the high- 

 est degree of precision. It is the experience of the writer, that one 

 hundred successive contacts may be made, in which another observer 

 at the microscope will be unable to detect the slightest deviation from 

 cons;^ancy. 



The stop method, also, has the great advantage over all other 

 methods, that it allows perfect freedom in the adjustment of the micro- 

 scope for focus, at any time during the comparisons. It is only 

 required that the stops shall remain fixed during the two or more 

 comjiarisons. 



But in order to meet the objection which is sometimes urged against 

 the certainty of actual contact, an electro-magnetic attachment has 

 been added to the stops H and H^ by which the plates K and I are 



