488 DUNCAN J. REID ON CEITICAL ILLUMINATION IN 



he is about to engage in critical work. Most microscope workers 

 see more easily with one eye, usually the right, than with the 

 other. For all adjustments of light, searching of the slide, and 

 for all work that is not critical or difficult, the left eye should be 

 used ; or else the critical observation postponed till the good eye 

 has recovered from its fatigue. 



Another important aid is the use of complementary colour 

 screens ; green for red-stained objects, yellow for blue, and so on. 

 The use of such colour screens will often enable one to see things 

 which might be otherwise invisible, besides saving the eye 

 considerably. At the Lister Institute, where much critical work 

 has to be done, Professor Minchin uses a liquid green screen, 

 consisting of a solution of a blue copper salt with picric acid. 

 This cuts out much or all of the blue, and practically all the red, 

 and if properly adjusted will also cut out the yellow, so as to give 

 almost monochromatic results. It passes a considerable amount 

 of light, and is very pleasant to work with. A very useful set of 

 screens are the M screens of Messrs. Wratten & Wainwright, 

 which give quite a number of useful combinations, but a blue- 

 green, yellow-green, blue and yellow will be found sufficient for 

 most purposes. 



I propose to treat the subject of illumination under the 

 following heads : 



1. The most suitable light. 



2. Collecting lenses. 



3. The principles of correct illumination : 



A. As regards the field. 



B. Filling of the objective with light. 



4. Condensers. 



5. Distance of lamp from sub-stage mirror. 



6. Critical and non-critical illumination. 



7. Working aperture. 



8. General arrangement of light and apparatus in high, 

 medium and low-power work. 



1. The Most Suitable Light. 



For visual work, I believe the kerosene lamp to be best, at any 

 rate for prolonged work. With a lump of camphor added to the 

 oil in the well, and with both the edge and flat of the flame 



