WENHAM'S REFLEX ILLUMINATOR. 161 



one that you know centres well with the objective to be 

 employed. With such a glass see if the central mark 

 placed on the instrument by its maker occupies the 

 centre of the field. If but slightly out of centre, a very 

 slight tipping of the instrument in its sub-stage fitting 

 will often be all that is necessary. If the stand has 

 centring screws these will come well into play. .We 

 have found it desirable to secure perfect centring, and 

 at the same time make sure that the plane face of the 

 facet is parallel to the right-hand edge of the stage, and 

 adjacent thereto. Having placed a small drop of gly- 

 cerine (which is preferable to water, as it does not dry) 

 on the top of the facet, and having placed the object 

 and slide in position, the " reflex " is made to approach 

 and the glycerine to contact the slide. It is better not 

 to allow the top face of the instrument to actually touch 

 the under surface of the slide, still keeping it, however, 

 so close thereto that the drop of glycerine shall be 

 considerably flattened. 



We are now ready to screw on the object-glass, and to 

 make immersion contact with the cover. For illumina- 

 tion we need one of the lowest and smallest kerosene 

 hand-lamps that can be found. If the stand is small 

 and low, it will become necessary to place it on a suit- 

 able box, the tube inclined to a convenient angle. Re- 

 moving the mirror, use the light direct from the lamp 

 keeping the latter for the present in nearly or quite a 

 central position. If, on looking through the eye-piece, 

 there be found a lack ot illumination, the trouble can 

 be rectified by moving the lamp slightly. Having 



11 Microscopy. 



