USE AND MANIPULATION OF MICROSCOPE 171 



but in any case, they should be so arranged that they may be 

 removed and replaced in their proper position at will. The 

 iris-diaphragm should now be closed as far as possible, and if 

 all is in correct alignment on viewing the illuminant through 

 the eye-piece cap, the closed sub-stage iris, and the extra inter- 

 vening diaphragm, a brilliant point of light should be seen. It 

 is obvious that if there is any want of centration between these 

 three small apertures, no light can pass, and it will be 

 found extremely useful to devote some attention to this point 

 at the start to ensure that such adjustment is exactly 

 carried out. 



This having been done, the auxiliary condenser may now 

 be placed in its position, which should be between the illu- 

 minant and the auxiliary iris-diaphragm. Assuming that this 

 collecting-lens is the one described by Messrs. Zeiss by the 

 Eoman figure I, the iris-diaphragm should then be placed 

 near to it. If reference is now made to Fig. 21a, it will be 

 seen that when using this No. I lens a convergent beam is 

 thrown into the back lens of the sub-stage condenser, and 

 this should be a sufficiently broad beam to fill completely 

 the back lens of the sub -stage condenser with light. This 

 lens is of the uncorrected plano-convex type, and the same 

 adjustment may therefore be made when using a bull's-eye 

 or any other similar system. 



An alternative method when a parallel or slightly con- 

 vergent beam is being used, is to place the subsidiary iris- 

 diaphragm as near as possible to the collecting-lens, close the 

 iris and focus its image by means of the sub-stage condenser 

 in the field of view ; the iris of course is opened again suffi- 

 ciently to fill the field of view when a photograph is to be 

 taken. A critical image of the iris is therefore obtained 

 instead of a critical image of the illuminant. It must be 

 admitted that while this does not exactly conform to the 

 methods advocated by some authorities, it does in practice 

 give extremely good results. For high powers this lens No. I 

 is sufficient to obtain a well and evenly illuminated field, even 

 with an illuminant of the smallest area, such as an electric arc 

 of small current-consumption. 



For medium powers the arrangement is seen in Fig. 21 b. 

 In this the No. I lens is taken much closer to the source of 



