ZOOLOGY AND BOTANY, MICROSCOPY, ETC, 



639 



The original form with the doublet is on the whole decidedly pre- 

 ferable, and forms a convenient pocket Microscope for field use in collecting 

 Infusoria, algas, &c. 



Rogers-Bond ITniversal Comparator.* — The special features of the 

 Universal Comparator (fig. 158), devised by Prof. W. A. Eogers and Mr. 

 G. M. Bond, are, as its name implies, the variety of the methods employed 

 and the range of work that can be done in comparing standards of length ; 

 each independent method, when carefully carried out, producing similar 



Fig. 158. 



results which serve to check or prove the comparisons. It includes a 

 method for investigating the subdivisions of the standard by comparing 

 each part of the total length with a constant distance, determined by two 

 adjustable stops. 



A heavy cast-iron base • is mounted upon stone-capped brick piers, 

 giving a permanent foundation to the apparatus. Upon this base, and 

 reaching frorn end to end, are two heavy steel tubes 3 in. in diameter, 

 ground perfectly straight, and made " true " by a system of local correc- 

 tions after they are firmly secured upon the bed-plate of the machine, the 

 object being to get a straight-line motion of the Microscope plate, which 

 slides freely on these true cylinders. Flexure of these cylindrical guides 

 is provided for, by lever supports at the neutral points. Fitted closely to 

 these guides, and outside of the range of motion of the Microscope plate, 

 are two stops, one at each end, as shown in the figure. The stops are 

 arranged to be adjusted at any desired position along the guides, and are 



* Description supplied by Prof. Rogers. Cf. also Proc. Amer. Acad. Arts, and Sci., 

 xviii. (1882-3) pp. 287-398 (7 figs.). Jonrn. Franklin Inst., cxvii. (1884) pp. 361-5 

 (2 figs.). 



