MEMORANDA. 255 
The Diagonal Scale.—Among the greatly increased and 
continually increasing number of microscopists, there may be 
some to whom remarks even of a somewhat elementary cha- 
racter may be acceptable. To such I would offer some sug- 
gestions on what, from my own experience, I conceive to be a 
very great desideratum. ‘This is a more extended know ledge 
of and practical agreement in microscopical mensuration. 
The various quantities used in defining the magnifying 
powers of lenses, as well as in speaking of the magnitude 
of images of objects, are to the uninitiated somewhat perplex- 
ing; but when the mind is trained to a just appreciation of 
standard measures, a moment’s consideration clears up what 
otherwise presents an apparently mystical and indeterminate 
aspect. 
On this account I would suggest to microscopists the general 
use of the diagonal scale, that is, of a scale of 12 inches, 
with the terminal inch divided both ways (ends and sides), 
the vertical lines being diagonal—a common Gunter’s scale 
at once provides the requirement. Why I suggest one 
of 12 inches is because it is portable and supplies a measure 
also for the determination of a 10-inch or other elevation in 
using the camera, Se. 
The inch being usually divided into 100th or 1000ths, I 
shall refer to these as the 100th or 1000th scales. 
If, therefore, the 100th scale is implied, the numbers on 
the line a B will indicate the parts of a hundred in tens for each 
line, and the numbers on the line Bc will indicate units. 
Thus, supposing I wished to know the exact measure of a mag- 
nifying power of 95, by placing one leg of the compass on the 
point a, and extending the other to the figure 9 on line aB, 
the space equals 90; then, by following down the lines dia- 
gonally towards c, until opposite the figure 5 on the unit 
line, I have a standard measure corresponding to a power of 
95. Every deviation from this must be accounted erroneous. 
If, again, I wished to know the measure of space on the 
1000th scale of a magnifying power of 450; then, what were 
before considered tens and units are now regarded 100ths 
and tens; thus, from the point a, to figure 4 on the line a B, 
equals 400, andi as in the former example, the distance on the 
diagonal to opposite the figure 5 on line Bc, equals 50, or 
450, the measure required. 
It will also frequently occur, that ten times this quantity 
may be needed, or ten spaces of 1000 inch scale, magni- 
fied in the same ratio; then, taking as above 450, add a 
cipher making 4500,—the 4 will then indicate clear inches ; 
the 5 on line 4B will equal hundreds; and, if tens are wanted, 
