MICROSCOPIC ILLUMINATION. 87 



iittings as the more highly corrected oil-immersion condenser. 

 The field illuminated by this system is large enough for quite 

 low-power work, and is found to be convenient, because the 

 aberrations which are obviously present do not matter for such 

 purposes. Of course in cases where a low-power lens is required 

 to do its best, the use of the more perfect condenser lens system 

 would be clearly indicated. 



Ainslie has described very completely the principles under- 

 lying correct microscopic illumination. He has pointed out 

 that the disadvantage of method 1 lies in the fact that the 

 illumination of the slide is not uniform, and that this may be 

 a serious objection for photography. There is the further 

 disadvantage described by him, and mentioned earlier in this 

 paper, that since the illumination cannot be restricted to the 

 portion of the field corresponding to the photographic plate, there 

 is some reduction in the brilliance and crispness of the image 

 owing to the scattered light. Ainslie points out that method 2 

 does not possess these disadvantages, for the illumination of the 

 field is uniform, and its extent may be restricted at will by the 

 iris diaphragm on the bull's-eye. He shows, however, that it has 

 the disadvantage that the aperture of the substage condenser 

 — and therefore that of the objective — may not always be uni- 

 formly illuminated. In many cases it will be more brilliantly 

 lit at its centre than at its edges ; which results, when a solid 

 cone of illumination is in use, in coarse details being more 

 brilliantly represented (since they require rays of small N.A.) 

 than fine details. The reverse is sometimes the case, i.e. edges 

 more brilliant than centre ; which causes the fine details to be 

 unduly prominent. I wish to point out that this difficulty may 

 be almost eliminated, at the expense of a certain amount of light, 

 by placing a slip of finely ground glass between the iris and the 

 bull's-eye. The illumination of the aperture of the condenser 

 becomes almost uniform under these conditions. Ainslie has 

 himself pointed out that unless the largest possible illuminated 

 area of the object is imperatively required, it is generally possible 

 to select a small uniform area in the light-source, and to project 

 this on the aperture of the substage condenser iris so as to fill 

 it completely. This may require a bull's-eye of very short focus, 

 and therefore very close to the light source ; obviously undesirable 

 with an arc lamp in use on account of the risk of overheating and 



