DESCRIPTION OF THE ACHROMATIC MICROSCOPE. 135 



nothing - further is necessary in this respect; for when 

 the object is brought into the focus of the microscope, the 

 illumination is sure to be correct. 



Persons who have never viewed opaque objects with 

 shallow or medium power object-glasses, and silver 

 cups of proper diameters and curvature, are not aware 

 of half the beautiful effects resulting from the achromatic 

 construction. And as for objects that are suited to this 

 method of illumination, they are almost infinite in num- 

 ber; and, consequently, a greater fund of instruction and 

 pleasure is derivable from it than can be obtained with 

 deeper powers, where the objects which can afford 

 amusement are much more limited. 



Sect. 14. Stops or Diaphragms are used for the purpose 

 of limiting the illuminating pencil of light, and may be 

 described as being small tubes with apertures at each of 

 their extremities. These are made to slide into sockets, 

 which screw into orifices formed to receive them at the 

 under side of each of the stages of the microscope. 

 They are sometimes furnished with a wheel of apertures, 

 similar to that shewn in the plate, fig. 24 ; and some- 

 times these sliding tubes have a small condensing lens, 

 such as that exhibited in the Microscopic Cabinet, page 

 1J0. By having the stops or diaphragms in the form of 

 tubes, the effect they are designed to produce may be 

 varied, if necessary, by sliding them up and down within 

 their sockets. Those possessing the lens, and intended 

 to act on Dr. Wollaston's principle, require this adjust- 



