312 J. BURTON ON A METHOD OF MARKING A GIVEN OBJECT. 



accurately placed in the centre of the field as possible ; then sub- 

 stitute for this power, preferably a water- immersion objective, 

 say T V^h, put on the front lens a small drop of water and care- 

 fully focus. It is necessary that the slide should not be moved 

 after contact is made, as it is desirable to keep the drop of water 

 as small as possible. When the object is recognised and is in the 

 centre of the field, raise the microscope tube rather sharply and a 

 small circular spot of water will be left on the cover-glass right 

 over the desired place. Now stain this spot with water-colour as 

 in the other case 1 always use the carmine kept for feeding 

 infusoria, etc., but any colour will do. When this is dry the 

 slide may be roughly examined and the object will be seen 

 through the coat of colour, which for this purpose should not be 

 too thick. If it be rightly placed, proceed as before, putting a 

 fine ring of suitable size round the spot with some dark cement, 

 and when this is dry carefully clean off the colour, and the 

 arrangement is complete. Water-immersion lenses are not very 

 commonly used now, and if the microscopist does not happen to 

 possess one, an oil-immersion may be used instead, but obviously 

 it must be used with water, not oil ; but this will give a sufficiently 

 good image for our purpose, which is merely to recognise the 

 specimen for marking, not to examine it. If an oil-immersion be 

 not available, any close- working objective, say gth inch or even 

 -g-th inch may be used, but it is necessary that the front lens be a 

 small one, so that the spot of water placed by it should be as 

 local as possible. 



There are, of course, some difficulties ; the chief is, that objects 

 mounted in glycerine as mine usually are are somewhat 

 liable to move if at all roughly handled, and may work 

 out of the circle ; but with balsam or glycerine jelly mounts, or 

 even a shallow glycerine one, there is little danger of this. If a 

 turn-table is not in the outfit of the experimenter, a sufficiently 

 good circle may be drawn by hand, or a line drawn to indicate 

 the position, or, as has been suggested, the barrel of a mapping 

 pen or similar object may be used. But the first great difficulty 

 is always to indicate the exact spot it is desired to mark, particu- 

 larly if the object is a very minute one, and that is got over with 

 facility by the method indicated. 



