IN TROD UC riON. 



xxni 



finger can be used to move the slide on the stage. In this way all unnecessary fatigue of the 

 arm is avoided. In Swift's microscope it is glass-plated. 



The diapliragm is placed under the stage, and consists of a circular plate, with a graduated 

 series of round apertures in it. When using a high power, turn round the diaphragm until 

 a small aperture usually the second smallest is directly under the hole in the centre of the 

 stage. A small catch indicates when this is accomplished. When using a low power, employ 

 a large aperture of the diaphragm. 



On the more expensive microscopes small stops, or tubes with graduated apertures, are 

 supplied with the lenses. They are slipped into a hole in the stage 

 from below, but care must be taken that they are accurately centred, 

 so that their apertures are exactly under the objective. 



The illumination. This is accomplished by means of a mirror 

 placed under the stage. Never employ direct sun-light ; rather 

 employ light from a white cloud. In selecting a site to work with 

 the microscope, try to secure a north light. 



ARTIFICIAL ILLUMINATION. 



Under certain circumstances it is necessary to use a lamp 

 which burns either gas or paraffine. I find that an ordinary gas 

 jet, fitted with a Sugg's burner, surrounded by a chimney of 

 pale blue glass, answers admirably. Figure 7 shows a form of 

 microscopic lamp which burns paraffine. It is also very convenient. 

 The light is better when it is transmitted through a light blue glass 

 funnel. It is made by Swift. 



An inexpensive convenient light is obtained from an ordinary Fig. 7. MICROSCOPIC PARAFFINE 



paraffine lamp with a flat wick. 



LAMP (Swift). 



CLEANING OF THE LENSES OR OBJECTIVES. 



The lenses are optically the most important part of a microscope. Never unscrew the 

 separate parts of the objective or lens. If there be any dimness, it is sure to be due to dirt 

 on the outer lenses. Carefully rub the lens with a piece of chamois leather, which should be 

 kept ready at hand. A convenient way is to tie it to the stand of the microscope. If a drop 

 of dammar chance to be on the lens, it must be cautiously removed by applying a drop of 

 clove oil and removing it quickly with chamois leather. It is necessary to remove the clove 

 oil at once, as the glasses which compose the lens are soldered together with Canada balsam 

 which would be dissolved by the clove oil. A dilute solution of ammonia applied to the lens 

 with chamois leather or cotton wool is an excellent method of removing the greasy film which 

 sometimes coats glass. The surface of a freshly-fractured piece of elder pith answers the 

 same purpose admirably. 



METHOD OF FOCUSSING AN OBJECT. 



It is extremely desirable for the student of histology to begin at once by learning to keep 

 both eyes open when examining an object. After putting on the proper lens, arrange the 

 mirror under the stage so as to direct a beam of light up the tube of the microscope and 

 illuminate the field. See that the field is brightly illuminated, and that no specks are present. 



