40 



THE MICROSCOPE 



Parabolic 

 reflector. 



Sorby 

 reflector. 



Glass 

 reflector. 



Vertical 

 illiiminator. 



Fig. 30.— No. 3360, 

 Parabolic Reflector. 



even when a moderately high power, such as a 1/6-inch (4-nim.) 

 is used, because the band of light is sufficiently narrow to be 

 directed through the small working distance between the object 

 glass and the object. This method is particularly useful for the 

 examination of alloys of metals or substances with fine laminae, 

 as the heavy shadows shown by such oblique illumination 

 indicate the character of the structure. 



Another method of illuminating opaque objects is by means 

 of a silvered parabolic mirror, which can be attached to either a 



IJ-inch (40-mm. or 32-mm.) or a 2/3-inch 

 (16-mm. or 14-mm.) object glass. The front 

 lens of the object glass being removed, the 

 tubular portion of the reflector can be slid 

 on to the cylindrical part of the object glass, 

 which is of a standard size. This may be 

 lightly clamped in position by the milled 

 head and the front lens replaced. The 

 object glass having been screwed into the 

 microscope and focussed, the reflector, which 

 has an adjustment up and down, should be placed so that 

 its lower edge almost touches the object. The light should 

 then be directed by a bull's-eye condenser in a horizontal 

 direction parallel with the stage, so that it illuminates the 

 whole of the reflector (Fig. 30). The reflector condenses it to 

 a focus on the object, and a slight movement of the reflector 

 up or down or a slight turn will give the best result. This 

 produces a very brilliant illumination, and as the light falls upon 

 the object from a large number of directions, the shadows 

 produced are not, as a rule, misleading in interpreting structure. 



Mr. Sorby devised an addition to this reflector, which 

 can be used with the IJ-inch (32-mm. and 40-mm.) object glass, 

 which consists of a small, flat, silvered mirror which swings in and 

 out of the optic axis, and when it is in position it covers half 

 the object glass. It reflects a beam of 

 light directly downwards upon the object, 

 which illuminates it in such a manner that 

 no shadows are produced. 



A modification of this apparatus is also 

 used for metallurgy in which a thin transparent 

 plate of glass is placed at 45° between a low- 

 power object glass and the object (see Fig. 31). 

 This has the advantage that it does not reduce 

 the aperture of the object glass. It can be 

 used with either a IJ-inch (40-mm. or 32-mm.) or a 2/3-inch 

 (16-mm.) object glass. 



For the illumination of opaque objects viewed with high 

 powers, a system was first invented by Mr. Richard Beck in which 

 a thin glass disc was placed behind the object glass, and a beam 



Fig. 31. —No. 



3362, Thin Glass 



Reflector. 



