TnE Microscope. 49 



Professor Rogers' micrometer rulings, both on metal and glass, 

 are well known to microscopists for their accuracy as regards divis- 

 ions, and also for the character and beauty of the lines. His divid- 

 ing engine with which these rulings are made, the construction of 

 which consumed several years, is a marvel of mechanical skill and 

 ingenuity. It is at present run by a small electric motor, and when 

 once adjusted and started it continues its work of ruling and spacing 

 by automatic arrangements. The lines, which are traced by the edge 

 of a split diamond, are spaced off by means of a large screw, the 

 trifling errors in its thread having been accurately determined. 



This screw, which was made by the Waltham Watch Company, 

 necessitated months of experiment. The company was soon able to 

 attain a high limit of precision in the construction of a screw having 

 a working length of nearly foiu' inches, but to make one with a work- 

 ing length of half a meter, was found impossible by any of the ordi- 

 nary methods of construction and correction. After spending nearly 

 two years in unsuccessful efforts, it was decided to adopt what is 

 known as a sectional screw. Threads were cut from the same part of 

 the leading screw upon ferrales 1^ inches in length, which were after- 

 wards placed on a cylindrical shaft in such a way that the threads of 

 adjacent ferrules would match. The screw as thus completed is 

 undoubtedly the most perfect of its kind ever made. 



As an outgrowth of this undertaking we now have what is 

 known as the Rogers-Ballou process, whereby a continuous and 

 practically perfect screw can be cut ' almost any desired length. A 

 screw made by this process for the dividing engine of Cornell 

 University, was cut and ground in 27 hours from the time the first 

 tracing of a thread was made;* at that time it was found to be per- 

 fect for about 20 inches. 



Professor Rogers has published over forty monographs on 

 scientific subjects. Among those of peculiar interest to scientific 

 workers with the microscope are " A study of the problem of fine 

 rulings with relation to the limits of naked eye visibility and micro- 

 scopic resolution ;""j" "On the conditions of success in the construc- 

 tion of standards of length, and their subdivision into equal parts;" J 

 "A study of centimeter A." "A critical study of the action of a§ 

 diamond in ruling lines upon glass and metals." ^ 



In 1880, Professor Rogers was made a Fellow of the Royal 

 Society of England, and two years ago was elected an Honorary 



* Transactions Am. Soc. Mechanical Engineers, Vol. V. 

 t Am. Monthly Microscopical Journal, 1885i, p. 165. 

 t Proc. Am. Soc. of Microscopists, vol. iv. p. 231. 

 § Ibid, 1883, p. 184. t Ibid. 1883, p. 149. 



