240- SUMMARY OF CURRENT RESEARCHES RELATING TO 



For the back combination of the objective to be uniformly filled 

 with light at full aperture we must have the following conditions : — 



(a) That the S.S. condenser should be capable of focusing on the 

 plane ©f the object under examination an aplanatic cone of light, and 

 should have a N.A. at least as great as that of the objective in use, and 

 preferably a little more. 



(b) That the image produced by the beam of light coming from the 

 lamp, through the collecting-lens system, and focused on the back of 

 the iris of the S.S. condenser, should be large enough to completely fill 

 the opening necessary to allow of full aperture with the objective in use. 



(c) That the S.S. condenser, the source of light, and the several 

 parts of the collecting system in use shall be all exactly centred with the 

 objective, and with one another. 



AVe shall now proceed to consider in detail some methods of obtaining 

 critical and effective illumination, with different powers of objectives, 

 under the conditions I have laid down, using a Nernst filament lamp. 

 These methods are suitable for the condensers employed by me (Watson's 

 parachromatic dry condenser, and Beck dry and immersion achromatic 

 condensers), and might have to be modified for other S.S. condensers. 



The lamp I keep constantly at 21 in. from the substage iris, as that 

 gives plenty of room for apparatus, and is also the distance which meets 

 the conditions of illumination best. 



For the collecting-lens system I use a Nelson lens, which is placed 

 quite near the lamp, and can be used as a collecting-lens to produce 

 converging rays, or as a collimating-lens, to produce parallel rays, as 

 may be required, and also certain supplementary lenses, which will be 

 detailed under Methods. 



Methods. — "With the high powers (from 4 mm. and upwards), taking 

 as a type the 4 mm. dry apochromat of • 95 N.A., I employ what might 

 be called the Xormal Method of Illumination, and 1 shall therefore 

 describe it as minutely as possible. 



Having arranged the Microscope in the horizontal position, with a 

 dry condenser of I'O N.A. in the sub-stage, and with the mirror swung 

 aside and the lamp at 21 in., and as nearly as possible in a line with the 

 tube of the Microscope, using No. 4 compensating ocular, we proceed as 

 follows : — 



1. Focus the objective on the object. 



2. Close the S.S. iris, and bring it into view in the field, by racking 

 the sub-stage condenser up or down, as may be required, and centre it. 



3. Open up the S.S. iris, and focus the filament of the lamp, and 

 centre it by raising or lowering the lamp, and by moving it from side 

 to side, as may be required, until it is quite in the centre of the field, as 

 seen by keeping one's eye on the ocular. 



4. Place the Nelson lens in front of the lamp, and adjust it as a 

 collecting-lens, so as to throw a sharp, enlarged image of the lamp fila- 

 ment on the back of the S.S. iris, and adjust it roughly to the proper 

 height, whilst keeping one's eye on the back of the sub-stage. 



5. Interpose water-tank between the Nelson lens and the Microscope. 



6. Close the Nelson lens iris, and centre it by moving it in the 

 required directions, as seen by looking through the ocular, and then, 



