336 ACCESSORY APPARATUS 



3. A stop of paper or varnish should never be placed behind an 

 object. 



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

 often most useful to examine an opaque object afterwards by trans- 

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

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

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

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

 on it centrally on the ' turn-table,' J and this may be placed under 

 the slide when 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 be examined either with the Lieberkiihn or by directly 

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

 object may 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 ^th f an ^ ncn would prove most suitable, and for 

 the diameter of the cover-glass J of an inch would be 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 cover-glasses such as those ^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. 



Lieberkiihns can be 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 

 reflected on the object. 



The light suitable for illumination by Lieberkiihn may be either 

 the flat of the lamp flame, reflected by the plane mirror, or the edge 

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

 reflected from the plane mirror to the Lieberkiihn. 



There is one other kind of reflected illumination em- 

 ployed, produced by the vertical illuminator, which, although it 

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

 from the employment of immersion lenses. The earliest device for 

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

 Geneva, U.S.A. 



The principle of this illuminator is to employ the objective as 

 1 Chapter vii. 



