ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



641 



of the two standards, since tlie distance between the stops may be con- 

 sidered constant for the short interval of time required to make the com- 

 parisons. It is the experience of Prof. Eogers that the precision of the 

 contacts is about four times as great as that of making coincidences between 

 a line of the scale and the micrometer line of the Microscope. The experi- 

 ment of making one hundred successive contacts and coincidences has been 

 frequently made without observing a single instance in which a variation 

 from constancy under a 1/4 objective could be detected. 



In the employment of the " two Microscope method," the comparator 

 has a convenient auxiliary attachment for observing the graduations when 

 the graduated surface is in a vertical plane, according to the method first 

 used by Lane of the U.S. Coast Survey. 



A modification of this form of comparator, made by the Ballou Manu- 

 facturing Company, of Hartford, Conn., from the plans of Prof. Eogers, for 

 Prof. Anthony, of Cornell University, is shown in fig. 159. The instrimaent 

 is mounted upon a single heavy base. Though not having the range of 

 motion in the adjustable supports for the standard bars possible with the 

 original comparator, it possesses all of the conveniences for rapid adjust- 

 ment and accuracy of movement. The right line motion of all moving parts 

 longitudinally is governed by heavy cylindrical guides, and the same 



Fig. 159. 



method of the " stops " is used in the comparison of either line or end 

 measure standards of length. In this form of the comparator an efibrt was 

 made to reduce the cost of construction without impairing the efficiency 

 of the apparatus. The reduction efi"ected in the cost was very considerable. 

 The instrument shown in fig. 158 cost 2500 dollars, while that shown in 

 fig. 159 cost only 800 dollars.* 



* Cf. also a paper by Prof. W. C. Unwin, " Measuring-Instruments used iu 

 Mechanical Testing," Proc. Phys. Soc. Loud., viii. (1887) pp. 179-84 (3 figs.). 



