THE MICROSCOPE. 119 
first. Some excellent rulings for this purpose were made years ago 
by Professor Wm. E. Rogers, at ;;/5;th in. which under the same 
circumstances would again double the reading; but the lines were, 
according to the writer’s judgment, too crowded for best effect. 
' More recently Prof. Rogers, before selling his ruling machine, 
introduced the novel form here shown. The whole scale is divided 
to ;} ths in., leaving the field nearly unobstructed and free from the 
confusing offect of crowded lines; and these wide divisions may be 
used (taking advantage of the middle lines in the subdivided spaces 
as’a means of reading halves) with low powers where close work is 
not required. But every fifth space is subdivided into ten, or 
zoyoths in., and by using these divisions for decimals, or these for 
units and the broad spaces for tens, one may gain the precision of 
the finer scale with almost the facility of the coarser. With a ;',th 
in. objective the coarse pee may be made, with a PTnORerEies use of 
of one of the fine bands for dente, Lotsa; in. A slight change 
of tube-length will give, with equal facility, a reading by {ths of a 
micron (), or even }ths , for easier relations to decimal notation. 
Thus an average human blood-disc may reach from the line 
marked ‘‘2” in the cut to about the 9th line in the fine band “a,” 
10 9 
TTT 
in. Likewise a disc of dog’s blood may reach from line ‘‘2” to the 
7th line of ‘‘a’’; of beef’s blood from ‘2’ to the 3rd of “a”; or of 
