82 THE Microscope. 
of thin glass may be cemented to the slide, or short pieces of 
hair or narrow pieces of paper may be placed under the cover. 
This will prevent the extrusion of the liquid to such an extent 
as only to permit of the examination of a film so thin as to con- 
tain no morphological elements; which of course may be a 
‘treason why tube casts are not seen when really existing.” 
Now, the magnifying power to be used for the examination 
is or should be a matter of some consideration. “The books” 
will frequently recommend that from three hundred to five hun- 
dred diameters be used, and as a rule, ihe “ family physician’s” 
highest authority will be ‘‘ the books.” 
Just now, I will not contradict “ the books,” but will advise 
the initial microscopist, of a medical turn of mind, to try var- 
ious plans; it being well known that after awhile an idiocratic 
condition will be developed which may be said to be immutable 
in each particular case. It may be well, however, to begin with 
a good one inch objective. Now, a“ good one inch objective ”’ 
does not necessarily mean. a lens of extremely wide aperture 
(and perhaps poor definition), but a lens wedl corrected; and it 
is highly probable that no American maker will supply any 
such lens of less aperture than will easily show what (in this 
case) may be required of it. 
Now, if the slide be placed upon the stage and examined 
in the usual way, the probabilities are that nothing will be seen; 
but if dark ground illumination be brought into use, the pres- 
ence of tube casts will be indicated (in all probability) by a 
faint, indistinct line of light (almost a “bluish shimmer”), 
showing, in the field, the position of each cast. These appear- 
ances cannot always be trusted, as indicating casts, until the 
observer becomes used to them, for other substances may give 
very nearly the same conditions (as to color, etc.) and yet not 
be casts. However, at such a (doubtful) stage of the examina- 
tion it is well to use central light with a high power. This extra 
power may be obtained by a tor 4 dry objective of 100° air 
aperture; and this aperture will be found to warrant (so far as 
tube casts are concerned) a power of nearly a thousand diam- 
eters, if proper eye-pieces be used, and (as a matter of course) 
due regard is put upon conditions of illumination. 
In many cases, however, it may be necessary, from a desire 
