34 Gibbons^ on a New Method of Micrometry, 
diameter^ requires a more rigid material. One half of the 
width of these glass scales is covered with paper so as to 
render it opaque, and the scale is drawn over both the trans- 
parent and opaque portions ; thus they do not need any 
special light to be thrown upon them, but the divisions are 
visible by the diffused light, whether transmitted or re- 
flected. This of course is an advantage, as the image of the 
object is not impaired by extraneous light. The diamond 
ruling will perhaps hardly be sufficient for the glass, and an 
opaque pigment will make the divisions more distinct ; on 
the paper ink alone suffices. 
The delicacy of the process will be judged from the cir- 
cumstance that, in the pattern scale sent herewith, the 
1 -10,000th part of an inch is not only readily observable by 
the naked eye, but is even susceptible of subdivision ; for 
the eye is always capable of appreciating low fractional parts 
wherever it can distinguish (i. e., detect space between) two 
objects, such as the divisions upon a scale. I have measured 
blood-corpuscles with these scales with as much ease, and 
much in the same way, as I should measure a window-pane 
with a common foot rule. 
It is, in fact, the readiest mode of applying to the magni- 
fied image of an object a similarly magnified image of a known 
scale, permanently constructed. These scales, when con- 
structed for given multiples of diameters, will, of course, be 
applicable to any instrument worked with similar powers. I 
would suggest to microscope-makers the advantage which 
would result from their sending out with their instruments 
scales constructed on this plan. They may be cheaply made, 
and would always be useful. Nothing would be easier than 
to print on a sheet of cardboard a series of scales, ranging 
progressively from 25 (say) to 1000 diameters, i. e., scales 
equal in division to such multiples of the graduations whose 
names they bore. Thus on the scale marked "100" the divi- 
sions marked 1 -100th would be an inch in extent, and would 
be subdivided ; and so on with the others. These might be 
cheaply produced, and would be always valuable. For inter- 
mediate powers each maker might issue scales with every 
instrument. Even if variation of the lenses caused them to 
be only approximate in the special cases, the remarkable 
delicacy of the process will give sufficient accuracy for ordi- 
nary purposes, even after leaving a margin for error arising 
from want of uniformity between different sets of lenses in 
combination. But a person, who, being a manufacturer, re- 
quired a large number of such scales, could easily construct 
them proportionally from two or three data. 
