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44 THe Microscope. 
thick for ordinary use. -In a later communication I shall describe a 
device of my own to prevent the sweating above alluded to. 
Another disadvantage of micrometers ruled on glass is the fact 
that there is always more or less uncertainty as to their staying 
qualities for some time after they have been ruled. This, so far as 
I have observed, is peculiar to all lines ruled on glass ; for I have 
observed them not only in scales ruled by myself, but on those by 
Prof. Rogers and Mr. Fasoldt. I do not say that this is universal; 
but it happens often enough to make the possessor sad. The makers 
are not to be blamed for this; for it seems due to an infirmity of the 
material. The only remedy is to let scales on glass season for an 
indefinite time, like thermometers, betore issuing them. 
My own judgment is that the very best scales are ruled upon 
metal. These can be depended upon. I have never seen one dete- 
riorate by simple lapse of time. But these have their disadvantages. 
They cannot be used with transmitted light, as can scales ruled on 
glass. Still this difficulty is not insurmountable. I use up to 400 
diameters, the opaque illuminating objectives made by Bausch & 
Lomb, which give excellent results. With higher powers, up to +4, 
I have used with satisfaction Prof. Smith’s vertical illuminator, with 
a bull’s-eye condenser to concentrate the light. With a very high 
power, a ;|, Zeiss’, draw-tube drawn out full length, amplifyer and 
high eye-piecing, I have never yet on my standard centimeter on 
speculum metal by Prof. Rogers, been able to see anything but 
clear sharp edges to the lines, saving now and then a little pit in the 
metal. Of course I understand that no practical use can be made 
of so high a power. I refer to its use simply to show the charac- 
ter of the lines. Any one who has used a glass micrometer with 
very high powers will agree with me in saying that in this respect 
they are vastly inferior to those on speculum metal. 
In order not to change the tube-length, when measuring 
miscellaneous objects, such as blood-corpuscules, ete, I had Mr. 
Bullock make for me an adaptor or nose-piece of the same length as 
my Smith’s Illuminator, also made by him, which I screw on to the : 
front of the tube in place of the illuminator, when I desire to 
measure transparent objects. This sort of a combination is, in’ my 
judgment, the very best that can be used. Metal micrometers have 
the disadvantage, however, of costing more than scales on glass ; for 
such a scale should be ruled on a carefully prepared surface, which 
of course adds to the expense. 
Now as to covering micrometers, in consideration of the disad- 
