284 PROCEEDINGS OF THE AMERICAN ACADEMY 



entirely enclosing the comparators, is built in the central part of the 

 enclosed space. The microscopes of the comparators «re 90 centi- 

 meters in length. Only the micrometer screws project through the 

 upper wall of the copper room, so that the heat of the body of the 

 observer can produce but a very slight effect. The value of one di- 

 vision of the micrometer is 0.301 /x. 



There are two rooms of the same form and dimensions. One con- 

 tains the longitudinal comparator, on which the transfers are made, 

 and on which a series of comparisons is also made directly after the 

 transfers, the two bars remaining in the same relative positions. The 

 other room contains the transverse comparator, with which the final 

 and definitive comparisons are made. These comparators were built 

 by M. Froment. 



The transfers and comparisons are only made after the bars have 

 been in position, and subject to the same temperature, for a period of 

 forty-eight hours. All the transfers and comparisons are made in air. 

 The tracings are made between one and two o'clock in the morning. 

 Even then, the jar of passing vehicles is very perceptible. The dis- 

 turbance is, however, strictly local. No matter how great the trem- 

 ors, no permanent movement of the bars can be seen. 



In the expose of the condition of the labors- of the French Section 

 to Sept. 22, 1879, will be found a con^lete statement of the present 

 state of this great work. 



A third natural unit of length has been proposed in the length of a 

 wave of light of given refrangibility. It is extremely doubtful, how- 

 ever, whether this unit will ever come into extensive use. In the 

 present state of the measurements of wave-lengths, the total number 

 in a meter is known with a far less degree of exactness than can be 

 assigned to the comparison of different meters. Probably the wave- 

 length of the line D is as well known as that of any other line of the 

 spectrum, and yet the measures by different investigators show large 

 discordances. The meter as determined by different observers shows 

 the following errors when compared with the value given l>y Ang- 

 strom : — 



