288 SUMMARY OF CURRENT RESEARCHES RELATING TO 



prepared on the under surface of cover-glasses, commonly, when not 

 measured and assorted, so thick as to make examination with the above 

 most suitable objectives impossible. 



To avoid this difficulty I dry and stain the material on the slide, 

 drop homogeneous-immersion fluid upon the preparation, and lower the 

 objective into the drop. Homogeneous fluid replaces both the balsam 

 and the cover-glass with optical propriety. 



A twenty-fifth, which has been nearly useless over ordinary cover- 

 glass preparations, is now used with gratifying freedom in manipulation 

 over uncovered, but homogeneously immersed, slide preparations." 



" Back of the Objective and the Condenser."* — The following are 

 extracts from an interesting article by Mr. E. M. Nelson on this subject. 

 Observing that a condenser was described as a " fad of English micro- 

 scopists," he thinks it will be worth while to try and account for this by 

 no means uncommon idea. The task is not an easy one, for there are 

 many fallacies underlying this impression. 



" First, we have spherical aberration. Many objectives, both cheap 

 and expensive, are turned out full of spherical aberration. If more than 

 the immediate centre of these lenses is used, the object will be flooded 

 or drowned in light. 



There are two kinds of flooding with light : one is due to spherical 

 aberration, as above, the other to the too powerful illumination of the 

 object. This last, however, seldom obtains in the Microscope, but is 

 always made an excuse for the other. Suppose we have a first-rate 1/2 

 of 60°. This lens will not be performing at its best unless it is illumi- 

 nated by a solid axial cone of 60° from a condenser, the object being 

 placed in the apex of the two cones. 



Under these conditions, it is by no means necessary that the illumi- 

 nation of the object should be too brilliant for the eye. If it is, it may 

 be modified by blue or neutral tinted glass. If the lens is free from 

 spherical aberration, the image will be clearer and sharper than if the 

 cone were to be reduced by means of a diaphragm. But if the lens is 

 mediocre or inferior in its correction for spherical aberration, then the 

 image will be fogged, though not necessarily too bright for the eye. 

 This fog, however, will pass ofi", as the angle of the illuminating cone is 

 reduced by the diaphragm. 



Very many histologists, biologists, &c., prefer their Microscopes 

 without condensers, because they are unable to illuminate their objects 

 with cones large enough to develope, so to speak, the latent spherical 

 aberration in their objectives ; at the same time, they seem to be 

 unaware of the fact that neither can they develope the resolving power of 

 the lens. 



Secondly, low-angled glasses for penetration : this fallacy is hung 

 on a peg of physical truth — viz. that penetration is inversely propor- 

 tional to aperture. 



The continual parading of this truth, and the placing of it in undue 

 prominence in several well-known microscopical works, has wrought an 

 incalculable amount of mischief. 



It has not only held back the progress of microscopy, but it has 

 directed many earnest workers in the wrong way. 



* Eng. Mechanic, xlviii. (1888) pp. 236-7 (4 figs.). See also pp. 260, 277, 278 , 

 295, and 296. 



