336 ACCESSORY APPARATUS 



H. A stop of paper or varnish should never lie placed behind an 



object. 



Let everv opaque mount be also a transparent one, since it is 

 (il'trn most useful to examine an opaque object afterwards by trans- 

 mitted light. The stop should always be a separate one ; this may 

 In- a disc on a pin held in the sub-stage, or, what is still simpler, a 

 piece of model atelv thick cover' glass, cut to the 3x1 inch size, 

 oi- rather shorter, should have a small disc of Brunswick black put 

 on it centrally on the .' turn-table,' J and this maybe placed under 

 the slide \\hen the Lieberkiihn is to be used. There may be two or 

 three such slips with stops of different sizes; in this way every 

 mount may lie examined either with the Lieberkiihn or by directly 

 transmitted light, and of course by having a larger stop the same 

 object mav be examined by any kind of reflected light. Many a 

 valuable preparation has been spoiled by placing a stop on it which 

 cannot be removed. 



4. It would be a most appreciable benefit to the cause of micro- 

 scopy, as we have already hinted, if a uniform gauge of thickness of 

 slip and diameter of cover-glass were adopted. For the thickness 

 of the slip, the oiyth of an inch would prove most suitable, and for 

 the diameter of the cover-glass | of an inch would lie most con- 

 venient, and if the thickness of the cover-glass were uniformly from 

 006 to '008 the gain would be still greater. Certainly no mount 

 ought to be finished without the thickness of the cover-glass being 

 marked in diamond point upon it, and a narrow ring of shellac 

 cement should be put round every cover-glass where there is even a 

 probability that a homogeneous lens will be admissible in examining 

 the object mounted. 



Very minute coyer-glasses such as those j\ths of an inch in 

 diameter are to be wholly condemned. They do not allow the 

 conditions required by modern microscopy, being adverse to the 

 employment of oil-immersion lenses in anything like the most 

 efficient way. 



Lieberkuhns can lie used with objectives as high as ^ of an inch 

 focus of '77 N.A. For higher powers than this a perfectly flat 

 speculum may replace the conical form, being illuminated by a 

 condenser with a stop, and racked up well within its focus. The 

 oblique annular ring of light falls on the flat speculum, and is then 

 relied ed on the object . 



The light suitable for illumination by Lieberkiihn may be either 

 the Hat of the lamp Hame. reflected by the plane mirror, or the edge 

 of the llame, the rays being rendered parallel by a bull's-eye, and 

 rdled ed from the plane mirror to the Lieberkiihn. 



There is one other kind of reflected illumination em- 

 ployed, produced l.\ the vertical illuminator, which, although it 



has been in use for some years, has received an accession of value 



fr the employment of immersion lenses. The earliest device for 



accomplishing this was invented by Professor H. L. Smith, of 

 < Iciiev a, I .S.A . 



I'he principle of this illuminator is to employ the objective as 



1 < 'lupin- vii. 



