VERTICAL ILLUMINATOR 



337 



its own illuminator ; which Professor Smith did by means of a 

 speculum. A pencil of light was admitted from a lateral aperture 

 above the objective and then reflected downwards upon the object 

 through the lenses by means of a small silvered speculum placed on 

 one side of its axis. 



Messrs. R. and J. Beck, in place of a speculum, employ 

 a disc of cover-glass. The cover-glass is mounted on a pin, B, 

 fig. 284, in order that it may be rotated, and oblique light obtained 

 by the milled head,/, A, fig. 284. 



Powell and Lealand's metkod is to fix a piece of glass, worked 

 flat, at an angle of 45 to the optic axis, with a rotating diaphragm 

 in front of the aperture admitting the light. 



FIG. 283. 



FIG. 284. 



To use these instruments the edge of the lamp flame should be 

 placed in front of the reflector, so that the rays may be reflected on 

 to the back lens of the objective in a line parallel to the optic axis. 

 The distance from the lamp to the reflector must exactly equal the 

 distance from the reflector to the diaphragm of the eye-piece in a 

 positive eye-piece, or the eye-lens of a negative eye-piece, otherwise 

 the rays will not be focussed on the object. 



This illumination is only suitable for objects mounted dry on the 

 cover, and with immersion lenses. No good result was ever obtained 

 until the immersion lenses were brought into use, but it is now 

 largely used in the examination of metals. The microscope adapted 

 to its employment is shown in fig. 207. 



Of all the light which is caused to pass out of the front lens of 

 the objective, through the oil and into the cover-glass, that which 

 has an obliquity less than the critical angle for glass (41) passes 

 through the cover and object and is lost ; but all the light which is 

 of greater obliquity than the critical angle for glass is totally reflected 



