84 Our Present Medical and Students’ Microscopes. 
Sliding movement of outer tube, witb oblique slot. — Good lor 
cheap lenses of small angles. 
One-inch. — X 10 (with lowest ocular, X about 50). Should be 
about 25° angular aperture. 
One-fourth or one-fifth inch.— X 40 or 50 (with low ocular, X 
about 200 or 250). May be 75° to 85°. 
One-sixth to one-eighth inch. — X 60 to 80. (With low ocular, X 
about 300 to 400.) Should be 120° to 140°. Best added after a few 
years’ experience with the others. 
Magnifying Poweks. 
Quality more than quantity. 
Theoretical. — One inch X 10. Oculars and adjectives as above, 
X 50, 100, 200, 400, 800. 
Practical .— Should approximate to this. 
Accessories. 
Essential, and should be furnished free. 
A few glass slides and covers ; stage-jplates with ledge; pipettes; 
pliers ; and needles mounted in handles. 
Desirable, for medical and general use. 
Compressor ium, or animalcule cage. 
Curved scissors (sideways). 
Camera lucida, for drawing and micrometry* 
Stage micrometer. 
Graduated draw-tube. 
Cylindrical fitting, below stage, to use ocular for achromatic con- 
denser. 
Reagents. (In twelve one or two ounce vials.) 
Turntable, and mounting materials. 
Desirable, more for general than medical use. 
Condensing lens, on separate stand, for opaque objects, and for 
parallel light, &c. ; or condensing lens to limb or stage of instrument, 
for opaque objects ; or mirror on curved arm, to swing above stage, for 
* For occasional measurements the camera is entirely satisfactory. Where 
large numbers of measurements are to be made, this method is not sufficiently 
rapid and convenient. Dr. White’s micrometer, a semicircle of thin glass, graduated 
at the straight edge, and lying upon the diaphragm in focus of the eye lens, is 
most used by the writer for this purpose. With low powers the object is easily 
brought up obliquely to the required position ; for high powers the Jackson 
adjusting screw must be used, unless the stand have a mechanical stage. The 
simplest ocular micrometer, and best for general students’ use, is a circular disk 
cut. to fit, from the centre of a rather thin stage-micrometer graduated to hundredths 
or to two-hundredths of an inch, and lying, only when used, upon the diaphragm 
in the ocular like Dr. White’s form. This rather thick circle is more durable, 
more easily obtained and handled, and capable of giving unexpectedly good 
results. It should cost $2 or $3. 
