Tue Microscope. 63 
METAL MICROMETERS. 
M. D. EWELL, CHICAGO. 
| DOUBT not that many of your readers are familiar with the use 
of micrometers ruled upon metal; but as some may not be 
familiar with their use, I venture to call their attention to the 
fact of their very great superiority over glass. To say nothing 
of their greater durability, in point of clearness and sharpness 
of outline there is no comparison whatever between the two. 
Any one who has ever seen the lines of “Cm. Scale A,” or of the 
standards ruled by Prof. Rogers upon speculum metal, will have 
no doubt upon the question. With a high power the edges of 
the lines ruled upon glass appear rough and uneven; but I have 
never yet been able to find a power high enough to produce 
such an effect upon my speculum-metal centimeter ruled 
to so mm., though [ have examined it witha Zeiss 75, Bausch & 
Lomb amplifier and in solid eye-piece, with the draw tube 
drawn out to its greatest length. In using this scale I usuaily 
use a splendid 4 opaque illuminator made by Bausch & Lomb ; 
with the high powers I use one of Prof. Smith’s vertical illumin- 
ators, which gives excellent results. For use on metal plates, 
however, I have never seen anything quite equal to the opaque 
illuminators made by Messrs. Bausch & Lomb, which have 
absolutely nothing to be desired as to illumination, and which 
are much easier to manipulate than the ordinary vertical i!lum- 
inator, though the management of the latter is not difficult, 
nothing further being necessary than to throw the light from a 
lamp through the diaphragm on the side of the illuminator by 
the use of a bull’s-eye condenser and to use as small an aper- 
ture for the admission of light as possible. 
ASTIGMATISM AND ITS RELATION TO THE USE OF 
OPTICAL INSTRUMENTS FURTHER CONSIDERED. 
BY. E GUNDLACH, ROCHESTER, N. Y. 
N the February number of Tue Microscopr I find a some- 
what extended criticism upon my article on “Astigmatism ” 
which appeared in the preceding number of the same journal. 
J feel under obligations to my esteemed critic for the able man- 
ner in which he discusses this important subject, and am bound 
