Illumination and Illimiinating Apjjaratus 105 



found that the bull's-eye has to be placed rather farther from 

 the lamp-flame (^ inch) when an ordinary glass chimney is 

 used than with a metal chimney and mounting slip of glass- 



The Parabolic, or Side Silver Reflector. 



To enhance the effects obtained with opaque objects with 

 stand condensers the side silver reflector will be found very 

 advantageous. The arm on which this is mounted is 

 attached to either the stage or limb of the microscope, or 

 fitted between the nose-piece and the objective. The 

 reflector consists of a highly-polished silver parabolic 

 speculum. This reflector is placed by the side of the 

 object, and light is thrown from the lamp through the 

 bull's-eye on to its centre, and then thrown by the reflector 

 on to the object. Most brilliant opaque illumination may 

 be obtained by this means. 



The Vertical Illuminator. 



The merits' of this piece of apparatus were but little 

 appreciated until the study of metal surfaces micro- 

 scopically was taken in hand. Its use had been almost 

 exclusively restricted to ascertaining whether the specimens 

 were mounted in contact with the under surface of the 

 cover-glass — information which is of importance when an 

 oil immersion objective is being employed on an object 

 mounted dry, for if the specimen is not adherent to the 

 cover-glass it cannot be seen at all with this illuminator. 

 It is also used for the resolution of the markings on diatoms 

 that are mounted dry on the cover-glass ; in this case it is, 

 however, only of value with immersion objectives. 



In metallurgical work it has rendered permissible the 

 examination of surfaces of metals with high-power objectives. 

 The surface to be examined is highly polished and then 

 etched with acid, liquorice juice, or other medium, as will 

 be found described on page 187, and no covering-glass at 

 all is used. It is made in two patterns, one with a prism, 



