SUBSTAGE CONDENSEES, ETC. 95 



oblique light be employed false images can actually be shown outside a diatom, and 

 what can be more disturbing to the scientific mind than to see a photograph of an 

 object with a well-marked additional image of one of its own edges— perhaps covered 

 with dot^—lying actually outside the real image of the object itself? There are several 

 who have given as much attention and more to the use of both annular and oblique 

 light than ourselves, and have abandoned the use of both unless they can make them- 

 selves absolutely confident that what they see with its use is nothing more than an 

 emphasised and better defined expression of what less perfectly can be seen without it. 



If any one is desirous of studying the effects of the use of annular light, whether 

 visually or photographically, he may find the following method of much service to 

 enable him to ascertain the diameter of the central stop to be placed in his condenser 

 for each objective. Having centred the light, look down the tube 

 after removing the eye-piece, and close the iris until it just equals the 48 

 aperture of the objective, which is known by seeing the iris com- 

 mencing to narrow the aperture of the posterior lens. Measure this 

 aperture of the iris directly with compasses, and make the stop for 

 the lens under consideration tV of that diameter. If this Httle stop 

 be suitably supported on thin arms and placed just above, or resting 

 on the edge of the iris itself, it will be found to leave an outside 

 ring of annular light about enough to fill the outer zone of the objective. 



For convenience, we may mention an arrangement that has been made at our sugges- 

 tion by Mr. Mason, optician, of Clapham. It consists of a ring, as the ordinary stops 

 are made, but where the three arms unite in the centre stop all the brass is filed 

 away, save a sufiicient amount to support a little projecting pin. Upon this pin any 

 of the accompanying discs of brass can be dropped— and several sizes are supplied. 

 This is shown in Fig. 48. 



When condensers were first introduced, and before theoretical considerations were 

 complete, they were racked up or down— that is to say, within or without the focus 

 — so that the image of the edge of the flame was avoided. An even illumination was 

 then thought to be of far more consequence. The "Carpenter" school used to say 

 the better effect was always produced by racking without the focus, whilst the 

 " Quekett " drsciple stoutly maintained it was just the reverse. Now-a-days we 

 know that for critical definition Sir David Brewster was correct when he pointed out 

 that the source of light should be focussed by the condenser on the object." Great 

 authorities like Mr. Nelson and others in the present day have proved this assertior 

 beyond all possibility of doubt. 



