STAINED AND UNSTAINED OBJECTS 91 



that is, getting the preparation or object into focus. The three ob- 

 jectives of a good modern microscope which are attached to the 

 triple nosepiece or revolver are parfocal and correctly and identi- 

 cally centred. The term parfocal may be best explained as follows: 

 Suppose an object is first focussed with the f-inch (16 mm.) or low- 

 power lens. The nosepiece is now swung around so that the 

 J-inch (4 mm.) higher power lens replaces the J-inch lens under the 

 tube. If the two lenses are arranged on the revolver in an absolutely 

 parfocal manner the ^-inch lens, after the turn is made, should be 

 strictly in focus. This, however, is rarely the case, and it is necessary 

 to use the fine adjustment to get a really sharp focus. So the term 

 parfocal has a relative value and meaning only. The term perfectly 

 and identically centred means that the centre of all objectives is in 

 the optical axis of the instrument, so that an object which is exactly 

 in the centre of one objective will also be exactly in the centre of 

 another objective if the latter is swung around to the place of the 

 former by turning the revolver or nosepiece. Here, likewise, instead 

 of absolute correctness only a more or less close approximation is 

 attained. Working with bacteria a f-inch and a y^-inch objective 

 are generally used. The ^-inch objective, except for the observation 

 of the ray fungus in pus, is generally employed in section work in 

 histology or pathology. 



In studying bacterial preparations the |- inch dry lens is first used 

 for a general orientation of the specimen and to pick out a spot which 

 is neither too much overloaded with bacteria nor overstrained. The 

 supply of organisms, however, should not be scanty nor understained. 

 The oil-immersion lens is now brought into use. It is not advisable 

 to swing around the T ^-inch lens and depend upon its being par- 

 focal with the f-inch lens. Often it is not, and by swinging it around 

 after the low-power objective has been in focus it may strike against 

 the cover-glass or some overhanging margin of the squeezed-out 

 Canada balsam. The glass may damage the expensive high-power 

 lens, and the balsam will certainly soil it and necessitate its being 

 cleaned. It is best to raise the tube before the oil-immersion lens is 

 swung into place. Good modern microscopes for work in bacteri- 

 ology and hematology are often supplied with an attachable mech- 

 anical stage, which permits of a systematic search over the whole of 

 the microscopic preparation. However, instruments for the use of 

 students in their laboratory training are rarely supplied with this 

 desirable accessory. There are also some devices called warm or 

 heated stages which permit the study of bacteria and other micro- 

 organisms at higher stationary temperatures. 



Stained and Unstained Objects. An important point in microscopic 

 work which the student should always recollect is the following: 

 When stained objects are examined the colored image shows better 

 the more powerful the illumination. Hence, in the examination of 

 stained preparations the iris diaphragm must be kept wide open. 



