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. When the section has been examined the oil must be removed from the lens. For this purpose a soft silk handkerchief or a special piece of chamois leather may be employed, and used very gently. If all the oil cannot be removed, the handkerchief may be moistened with a little absolute alcohol, and the lens hastily wiped. The alcohol must not be allowed to remain in contact with the lens as it is a solvent of Canada balsam with which the lenses are often cemented in position. [Illustration: Fig. 1.--Double or Triple Nose-piece.]