MICROSCOPIC ILLUMINATION. 83 



negyected. The disadvantages of the method are (a) that it is 

 not possible to limit the illumination to a portion of the slide, 

 (6) that small sources (such as an arc lamp) cannot be economic- 

 ally employed (so that the method is not good for projection). 

 The advantages of method 2, system A (2), is that illumi- 

 nation can be limited to a portion of the slide, thus reducing 

 stray light, and that small sources can be economically used. 

 The method has therefore important advantages for photo- 

 micrography and projection. The disadvantage of the second 

 method is that a highly corrected condenser lens system must be 

 employed if its full advantages are to be obtained. Slides of 

 different thickness then require a correction for tube length 

 when the condenser is used dry, or a change made in depth of 

 top lens when used with immersion oil. The first method 

 (critical light) has neither the advantage of simplicity possessed 

 by method 3, nor is it so good for photomicrography or pro- 

 jection as method 2, since the light cannot be restricted at will 

 to a portion of the slide. 



Consideration of the above leads to the following opinion. In 

 the ordinary use of the microscope, method 3 (opal glass) provides 

 an inexpensive and simple technique, quite adequate for histo 

 logical and medical purposes. Its results are equally good with 

 all powers. For research work with the microscope and for 

 projection and photography a more elaborate technique based 

 on methods 1 and 2 must be employed (the latter being preferable). 

 A convenient arrangement for research which has the advantages 

 of those methods, and provides ample illumination even for 

 high magnification, is shown in the diagram at (4). For projection 

 I have found most suitable the method called by Ainslie system 

 B. In this an image of the light-source is formed by a 

 chromatically uncorrected condenser in the plane of the iris 

 diaphragm, an image of which is focused on to the slide. For 

 closing this iris greatly reduces the intensity of the heat rays. 



Section VI. 



Practical Applications. 



I have developed and applied to my own microscope this last 

 system of illumination (method 4), since it forms a very convenient 

 arrangement. Below the iris diaphragm of the condenser is 

 mounted an auxiliary lens which removes the chromatic under- 



