Lecture I. 5 



into position under the body of the instrument, then the accurate 

 construction of the nose-piece and objectives secures that the 

 high power is exactly over the centre of the field previously viewed 

 by the low power and the object will be approximately in focus for 

 the high objective. 



Such exactitude of construction is not always to be found in 

 microscopes, so that certain precautions must be taken in order 

 to make sure to avoid injuring the high power, and spoiling the 

 preparation by dragging the lens of the objective against the cover- 

 glass. If by any chance the front lens should get smeared it should 

 be immediately cleaned carefully with a piece of clean linen. In 

 order to avoid any mishaps in turning on the high objective you 

 should proceed as follows : Raise the body of the microscope by 

 turning back the milled head of the coarse adjustment about one 

 quarter revolution then turn rouriti the nose-piece bringing the 

 high power into position, and almost completely close the dia- 

 phragm. The front lens should now be about 3-5 mm. above 

 the cover-glass. Put your eye to the eye-piece and while looking 

 down the now somewhat darkened tube rack the coarse adjustment 

 very slowly and carefully downwards. Soon hazy objects will loom 

 up in the field. Now cease using the coarse adjustment and 

 transfer your hand to the fine adjustment ; by moving this you will 

 quickly bring one of the cells into sharp focus. When it is in 

 focus carefully note the distance of the front lens from the cover- 

 glass by viewing it from the side. In order to be able to make 

 the change from low to high objective more speedily in future, 

 it is well now, without altering the focus, to switch back the low 

 power and note if it is still in focus ; or, if adjustment is required, 

 note if it is necessary to raise or lower the body. If no adjust- 

 ment is necessary, or if the tube has to be raised to bring the low 

 objective into focus, then you will know that when making the 

 change from the low to the high power in future you need not 

 screw back the coarse adjustment but simply switch on the high 

 objective after ascertaining the cleanliness of the cover-glass. If, 

 when turning back from the high objective, the low objective re- 

 quires to be depressed to bring it into focus, then you must always 

 raise the coarse adjustment before switching on the high power. 



When the high power is put into position one should try many 

 experiments with the mirror and the diaphragm in order to obtain 

 the most satisfactory combination of arrangements for the study 

 of each object. Above all, special attention must be given to the 

 fine adjustment. The high power has a very slight depth of focus. 

 That is, when it is in focus for any one minute object, objects 



