THE MICROSCOPE. 
469 
than half an inch are usually termed low powers, and they 
are made as low as two inches (2 -in. -power). If the focal 
distance is less than half an inch, it is a high power. Thus 
we have various powers, called respectively, two-thirds (of an 
inch), quarter- inch, eighth, and even sixteenth, though the last 
power is seldom used, and very liable to erroneous interpre- 
tation, even in the hands of experienced microscopists ; and 
indeed the eighth is only necessary for the more recondite' 
investigations. Useful powers, particularly for beginners, are 
the inch (fig. 3) and the half-inch, to which after a time the 
quarter-inch may be serviceably added. The magnifying- 
power of these objectives depends upon the character of the 
eye-piece, of which there are sometimes three descriptions used, 
called respectively, No. 1, No. 2, and No. 3 eye-piece, — No. 1 
having least magnifying power, and No. 3 the greatest. The 
two-inch objective with No. 1 eye-piece magnifies twenty times 
linear (or twenty diameters), — with No. 2 eye-piece, thirty-five 
diameters. The inch objective, with No. 1 eye-piece, magnifies 
fifty-five diameters, — with No. 2, one hundred. The half-inch 
with No. 1 eye-piece magnifies one hundred and twenty dia- 
meters, — withNo. 2, two hundred and ten diameters. The quarter 
with No. 1 eye-piece equals two hundred diameters, — with 
No. 2, three hundred and fifty, — and the eighth, four hundred 
and fifty and seven hundred and sixty respectively. 
In selecting one of these powers for the examination of any 
object, the observer must be guided by the size of the object. 
If of a considerable or appreciable size, it should be first 
scrutinized through a low power, which indeed may be found 
sufficient for the purpose, but if not, the next higher power 
may then be employed. If the object be very minute or 
imperceptible, the half or quarter-inch may be at once brought 
to bear upon it. And here let us remark, that in unscrewing 
and screwing on these objectives, some difficulty is apt to occur 
from the worm of the screw not easily slipping into the groove, 
particularly if the screw be a fine one; and the patience is some- 
times severely tested by the delay. It often happens that this 
difficulty may be at once surmounted by giving- the screw a 
turn in the wrong direction, by which means it is easily slipped 
into the groove, and may then be readily fastened. 
To get the proper focus with a low power, all that is required 
is to turn the large lilled-heads which move the tube of the 
microscope upon the rack-work, hence termed the coarse 
adjustment; but when high powers are used, more care is 
required in manipulation. The objective should be cautiously 
brought down by means of the coarse adjustment to within 
a short distance of the slide upon the stage, and then, with 
the eye at the eye-piece, and the finger upon the fine adjust- 
NO. iv. 2 K 
