Microscope.—This is not the place for a description of the microscope as an optical instrument, but some hints as to the selection of one may be found useful.
Showy microscopes with much brass work should be avoided, simplicity of construction being a great recommendation. The microscope should have a large heavy base, either of the horse-shoe or tripod pattern, large enough to afford a firm base when the microscope is tilted.
Mechanical stages are unnecessary and they add greatly to the expense, and very little to the utility of the instrument for ordinary histological work. Binocular arrangements also are of little use for this purpose.
The microscope should be provided with a coarse and fine adjustment, which should be most carefully tested before purchasing the instrument. They should work freely and smoothly, and the slightest turn in either direction should at once alter the focus.
There should be a reversible mirror, one side being concave and the other plane. The concave surface is the one usually employed, the plane surface being chiefly used in conjunction with the sub-stage condenser for the examination of micro-organisms. There should be an eye-piece of moderate magnifying power. Very powerful eye-pieces do not reveal additional details, but merely enlarge the image, and with it any defects that may be produced there by faults in the objective. Eye-pieces II. and IV. of most makers will be ample for most requirements.
Objectives.—These are the most important parts of the microscope, and the student will be well advised if he spends a little extra money to secure good lenses.
Most objectives and stands are now made with a universal thread, so that any objective will fit any make of stand. Many workers provide themselves with a cheap stand such as that supplied by Leitz, and then fit it with lenses by Zeiss, or other first class maker.
The most useful lenses are the 1 in. low power lens, and 1/5 in. or 1/6 in. high power, or No. 3 and No. 7 of Continental makers, or Zeiss’s A and D. A 1/2 in. lens will also be found very useful.
For minute work, such as bacteriology and blood investigations, higher powers will be required, 1/8 or 1/12 immersion lenses. These objectives come extremely close to the object, and very thin cover glasses must be employed. In order to avoid the refraction caused by the rays traversing the air between the coverslip and lens some immersion fluid is placed between the two. With some lenses water is employed, but usually an oil having the same refractive index as glass is used, and the one most generally employed is cedar oil (Zeiss prefers the oil from the species Juniperus virginiana). A spot of oil is placed with a rod just over the object to be examined and the objective carefully lowered by the coarse adjustment till it comes in contact with the droplet of oil. The focussing should then be managed with the fine adjustment only.