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XIX. — An Auxiliary Illuminating Lcnx. 

 By Edward M. Nelson. 



{Bead October 21, 1908.) 



An objection, repeatedly raised against my method of critical 

 illumination, ever since its introduction more than thirty years 

 ago, is that the image of the edge of the flame does not fill the 

 field. This is a great drawback to biologists in general, who use a 

 Microscope merely as a glorified magnifying glass, and prefer to 

 search over specimens beautifully clothed in the woolly garments 

 of diffraction fringes on a fully illuminated field, to any careful 

 examination of an object when seen in the image of the edge of 

 the flame. Some years ago I tried to remedy this defect of an 

 incompletely illuminated field by placing in the path of the 

 illuminating rays a small plano-convex lens at a distance of about 

 its own focal length from the edge of the flame. 



When this lens is properly adjusted it appears as a luminous 

 disk, and this disk, projected upon the plane of the object by the 

 substage condenser, yields a full and evenly-lighted field. This 

 method was, however, abandoned because any object partaking of 

 the nature of a minute lens, when examined upon this fully illu- 

 minated field, appears with a diminutive image of the edge of the 

 flame in it : whereas, when the same object is viewed in the image 

 of the edge of the flame, it appears as a small disk. This small 

 disk is, in reality, an image of the back lens of the substage 

 condenser. These phenomena may be studied upon an Actinocyclus 

 Balfsii, or other suitable diatom. 



Another, and more serious objection, is that unless care be 

 exercised, the W.A. may be considerably reduced. The state of 

 the case is this : — (1) If the auxiliary lens is focused upon the 

 edge of the flame, the rays which fall upon the substage condenser 

 are parallel. The substage condenser will, therefore, require to be 

 focused up a little nearer to the object. The W.A. will be of full 

 size, the field will not be illuminated by an even disk of light but 

 by a magnified image of the edge of the flame. (2) If the 

 auxiliary lens is arranged so that it will give an even disk of 

 light upon the field, it must be placed closer to the lamp flame. 

 A divergent beam will fall upon the substage condenser, which 

 must be focused down until the image of the auxiliary lens appears 

 quite sharp in the field. The W.A. will be reduced and the field 

 evenly illuminated. When the auxiliary lens has been placed in 



