324 STUDENTS’ MICROSCOPES. 
Glass, wagered a hand. — Cannot be too highly commended 
nary medica 
omen by hand (cloth-lined, or otherwise). Ag 
substitute. 
Plain. — Saves expense. Should have removable spring ¢ 
have glass or brass sliding-stage added. 
MIRROR. ; 
Must have lateral, vertical and two rotary movements, 
copes. Arol 
which carries it. Lateral movement by hinge joint at top of 
carries mirror: obliquity entirely independent of distance of 1 
object. 
Concave. — Indispensable. Should be nearly two inches ' 
Plane. — Very desirable and costs little. May be smaller. 
Fitting should be removable, ve raoi ligħt. 
c with apertures.— Essential. Should be closed betwe 
and stage to prevent accidental oblique light. 
“ Graduating” or “ Iris.”—~Incomparably best, but un 
sive. 
OCULARS (eye-pieces),. 
Negative i, always meant, unless otherwise 
(and performance) varies greatly, though seldom aus € 
Two-inch. — (The A or No. 1 of most makers.) 
use. Should always be negative. 
One-inch. — (Often named B or C, or No. 2 or 3.) X 
crometry, occasionally for observations, and general, 
denser. If willing to add $5.00 or $10.00, substitute # 
by European or American makers (superb field), or 
by B6ston Optical Works (field narrower, but pecul 
opsectives. (If extravagant anywhere, put it 
All above 50° angular aperture, should have ics 
All above about 75° nage have the same. 
Screw collar. — 
Screw movement ny outer tube.—Good for cheap 
Sliding movement of outer tube, with pb 
lenses of small angles 
One-inch. —X 10 (with lowest ocular, X about "i 
25° angular aperture. 
-One-fourth or one-fifth inch. —X 40 or 50 (with low 
One-sixth to one-eighth inch. — X 60 to 80. (With 
800 to 400.) Should be 120° to 140°, Best added a 
rience rience with the others 
* Magnifies, diameters. 
