ey 4 
THE MICROSCOPE. 45 
vantages incident to covered scales, I would recommend the use of 
a scale uncovered. If desired for use with a homogeneous immer- 
sion objective, it can be used with a large temporary cover, which 
ean be held down with a mere dot of mucilage or water, not enough 
to reach the lines. It should not be rubbed, but may be kept sufii- 
ciently clean with a camel’s hair pencil. I say sufficiently clean, of 
malice prepense. I now think that no one but an amateur with 
very little experience, will be annoyed by a little dust on a standard 
when used with a dry objective. If it becomes too thick, it can be 
removed with a camel’s hair pencil. If used with an immersion 
objective, of course the top of the temporary cover should be clean. 
I find a little dust a real convenience, as facilitating the finding 
and focusing of the lines. A really fastidious person should use 
“Centimeter A” for atime. Its surface, the last time I saw it, was 
in places seamed and furrowed like the track of a glacier. But 
enough of it is perfect for any sort of use, and its lines cannot well 
be excelled. Its correction for total length is very small, and its 
second millimeter has practically no error. 
Of course a micrometer in its ultimate subdivisions, such as are 
usually used in determining the value to be assigned to one division 
of the eye-piece micrometer, should have an error so small as to be 
practically insensible, or its error should be well determined. I 
have never yet seen, nor do I ever expect to see, a scale in every part 
absolutely free from error. I undertake to say that such a scale can 
not be made by any living man. But the absolute and relative 
errors of a scale can be determined within very narrow limits, and a 
scale can be made, the errors of whose ultimate subdivisions are 
practically insensible. Such a micrometer is practically perfect. 
In a future communication, should the subject be thought of sufficient 
importance and interest, I will describe the process by which any 
good observer, who is the owner of a filar micrometer, and who 
knows the correction for total length of his micrometer, and last but 
not least, who has sufficient patience, can determine the errors of 
any subdivisions small enough to be brought within the field of his 
microscope. 
Curcago, October 20, 1888. 
Leper has found that both an alcoholic and an aqueous extract 
of the streptococcus pyogenes aureus are capable of causing 
suppuration. He has isolated from these extracts a crystalline sub- 
stance, phlogozine, which is the active agent in pus formation. 
