200 Botanical Microtechnique 



ill mail) ways most desirable, binocular body has the eyepiece tubes 

 inclined. This permits the head to be held in a comfortable position 

 and greatly reduces fatigue. 



An important category of binocular microscopes utilizes matched 

 pairs of objectives. This type is customarily known as the dissecting 

 binocular or stereoscopic binocular. These instruments show true 

 perspective and depth. The image is erect, thus facilitating dissection, 

 isolation, and other manipulations of the object. The practical range 

 of total magnifications is from 10 to 150X- Two or more pairs of 

 parfocal objectives can be installed on a nosepiece of either the 

 revolving or sliding shuttle type. In one Spencer model a set of 

 objectives may be permanently installed on the objective changer, a 

 desirable arrangement for class use. For research work, each pair of 

 objectives may be obtained in a removable mounting, readily inter- 

 changeable on an objective changer, which, in the several makes is 

 either a rotating drum, a rotating disk, or a sliding shuttle. 



Several categories of noncompensating oculars are available for 

 twin-objective binoculars. The standard Huygenian type is the least 

 expensive and probably the most satisfactory for classwork. AVide- 

 field ocidars are well worth the greater cost. Two manufacturers 

 produce a good junior-wide-field ocidar, intermediate in cost and 

 performance between Huygenian and wide-field oculars. High eye- 

 point oculars also are available, but they recjuire that the eyes must 

 be held at restricted eye position, making these ocidars objectionable 

 to some workers. 



This chapter would be incomplete without a few words concern- 

 ing the durability and life span of the microscope. It must be obvious 

 that the period of service obtainable from a well-constructed micro- 

 sco])e depends uj)on the skill and care with which it is used, the 

 amount of use, and certain environmental conditions, such as atmos- 

 pheric conditions, extremes of temperature, and corrosive chemical 

 fumes. An outstanding illustration of durability is afforded by an 

 occasional microscope that seems to be in excellent mechanical and 

 optical condition after .10 xtais of continuous research service. On 

 the other hand, a classroom instrument may be in poor condition 

 after 10 years of use. Serious scratching and corrosion become CNident 

 first on the 4-mm. dry objective, the oil-immersion objective, and 

 on oculars, especially the type having a raised eye lens. The lower 

 power ol)ieclives should show no contac t wear or corrosion, especially 

 if the instrument has a safety stop on the liody tube. Examination 

 of large numbers of class microscopes has shown that the serviceable 



