828 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



skilful tands is by far the best arrangement of the two, the former 

 ingenious contrivance being suitable for one class of illumination 

 only. . . . We do not hesitate to recommend the achromatic con- 

 denser as being superior to any other method of substage illumination, 

 for in spite of all that has been both written and said in praise of the 

 swinging substage illumination, the practical results hitherto obtained 

 by it prove that its resolving capabilities have been greatly exagge- 

 rated, when compared with the performance of a high angle achromatic 

 condenser." 



Kelner Eye-piece and Equilateral Prism as a Meaus of 

 Illumination. — Mr, James Smith points out that whilst both the 

 Kelner eye-piece and the equilateral prism have long been in use for 

 illumination, the combinations he employs have, he thinks, some 

 novelty, and he thus describes them : — 



Fig. 190. 



Fig. 191. 



" It will be seen from Fig. 190 that I have divided the Kelner 

 eye-piece into two parts, the eye-lens A being in an outer tube C 

 and the field-lens B in another tube D sliding into the former. 

 When the field-lens is upwards and pushed up into the tube C the 

 eye-piece will be in its normal state, and act as an illuminator for 

 high powers (up to the -^q), as described and used by Dr. Beale ; but 

 when the field-lens is reversed as in the drawing, the two lenses are 

 then considerably separated, a much larger but more softened spot of 

 light is the result, and it can be used advantageously with low powers 

 even down to a 3-inch objective, giving a fully illuminated field. 



In combination with the eye-piece I use an equilateral prism A 

 mounted as in Fig. 191, turning on its axis and also at right angles 

 to it by means of a circular jilate B C. The prism has cemented to it 

 a piece of pale-blue glass E, and the whole slides on the mirror-arm 

 F in the usual way. The equilateral prism is convenient, as taking 

 the light from any side indifferently. When the light from the lamp 

 passes through the blue glass it is modified, and a fine white light is 

 obtained more like daylight, and the natural colours of delicate objects 

 such as diatoms and desmids in fresh water are beautifully shown ; 



