THE COMPOUND MICROSCOPE. 3 



stage (ST)*, upon which the micro-preparation is placed for exami- 

 nation, and can be, if desired, fixed in position by means of the 

 two clips seen on the upper side ; the use of these being essential 

 when the instrument is used in an inclined position, but undesir- 

 able when the microscope is used, as it should in general be, in 

 a vertical position this latter position being essential for the 

 safe use of many mounting fluids, and the most convenient when 

 rapidly adjusting or examining an object. From the upper part 

 of the pillar projects forward a bearer carrying the sheath (SH) 

 within which slides the tube (T), at the two ends of which are the 

 lenses used for magnifying the object. The lens, or system of 

 lenses (o), screwed in at the lower end of this tube, is called the 

 object-glass, or objective ; the system of lenses inserted into the 

 upper end is called the eye-piece, or ocular (E). The light is 

 directed upon the object, and through the lenses and tube of the 

 instrument, by means of the mirror (M), which is inserted by a 

 short unjointed arm into the lower end of the pillar, but can be 

 turned in two planes for any direction of light, from front or side. 

 In some microscopes the mirror arm is jointed, so as to admit of 

 the light being thrown obliquely upon the object, but for ordinary 

 student work this is rarely of any value. The light passes through 

 a circular perforation in the stage, under which is a tube or 

 sleeve, the condenser-tube (c T), in which can be inserted the 

 iris-diaphragm (i D). The plates of this iris-diaphragm can be 

 opened, narrowed, or nearly closed by means of a projecting lever 

 handle, shown in the figure, and thus the quantity of light thrown 

 upon the object can be regulated. l For a small extra charge 

 {10s.) the makers supply a condenser, fitting into the tube of the 

 iris-diaphragm under the stage, by means of which light can be 

 concentrated upon the object ; and the student is strongly recom- 

 mended to purchase this additional appliance, since by its aid, 

 and the regulation of the iris-diaphragm, a very wide range o 

 illumination becomes possible. Its value is very great in dull 

 weather. The mirror frame (M) bears two mirrors, that on the 

 one side being plane, on the other side concave. The former may 

 be used for work requiring the use of low magnifying powers only, 

 the latter for higher power work. With the condenser, the plane 

 mirror alone is used. 



1 In the cheaper forms of microscopes of many makers, instead of the iris- 

 diaphragm there is a fixed rotating wheel-diaphragm, with perforations of 



various sizes. 



