282/ Transactions of the Society. 



IX. — Critical Microscopy. 

 By Edward M. Nelson. 



(Read April 20, 1910.) 



Fi;OM time to time requests are received for information as to the 

 method of procedure for obtaining a critical image. As the pub- 

 lications upon this subject are much scattered — e.g. some being in 

 back numbers of the English Mechanic,* not always accessible, 

 and some in this and the Quekett Microscopical Club Journals, 

 while some are included in papers upon other microscopical 

 subjects — it is therefore not always easy to give correspondents 

 suitable references which they can find. Several times I have 

 been requested to publish a description of the whole method, so 

 that it may be readily available in a single reference ; to this I 

 now accede, the more readily as the method is so simple that it 

 can be explained in a few words, by which much future trouble 

 will be saved both to correspondents and myself. 



When, in the early seventies, I began microscopical work, the 

 leading Microscope makers supplied in the box with the Micro- 

 scope a whole drawer full of various illuminating apparatus, 

 which at that time was considered necessary in order to obtain 

 the best results. I well remember going through all these various 

 appliances and practising so as to be proficient in the manipulation 

 of each of them ; but when in 1875 the advantage of the large 

 axial cone was perceived, all this apparatus was abandoned, with 

 the exception of the achromatic condenser, a silver side reflector, 

 and a few lieberkiihns. 



It is now difficult for us to realise the position of " Microscopy " 

 at that date. Student's Microscopes, for example, hardly ever had 

 a substage-condenser ; it was considered right that they should 

 have at least a wheel of diaphragms, but some did not have even 

 that. An examination of twenty-three Student's Microscopes 

 of that time showed that only one was fitted with a substage- 

 condenser (Webster's) ; another, a Boss No. 3, had a fitting to carry 

 a substage. The better class of those instruments were capable of 

 having a substage fitted, but a substage together with a substage- 

 condenser was always looked upon as an accessory, much in the 

 same way as we to-day regard a polarising prism, or a revolving 

 selenite holder. An authority of that time writes : — " The stage 



* English Mechanic, xxxviii., xxxix., xl. (1883) No. 977 (1884) Nos. 980, 986, 

 1017, 1021, 1027. 



