On Ghost Images. By A. A. C. E. Merlin. 649 



manner ; for it is undoubtedly true that certain exceptional high- 

 power lenses will allow almost their full aperture to be utilised 

 with axial illumination without exhibiting fog or pale and defective 

 images. Unfortunately such objectives are extremely rare : they 

 " happen " and cannot yet be produced at will. 



Eegarding this question of the real utility of employing very 

 deep eye-pieces in certain cases with objectives of fairly high 

 optical index, I may mention that a non-microscopical friend, on 

 being shown some well stained bacteria under a Zeiss apochromatic 

 ■|- in. of N.A. 1"42, a Powell compensated 40 ocular, and the full 

 illuminating cone of the dry apochromatic condenser, without 

 knowing or understanding anything of the optical arrangement, 

 remarked on the clearness and distinctness of the forms and the 

 restful quality of the green light (Gifford's screen) to the eye. On 

 changing to the X 12 eye-piece my friend at once protested that 

 he could not see nearly so well or comfortably. One so often hears 

 that any tiling over a x 12 ocular is superfluous, that I had come 

 to regard my own predilection for high eye-piecing as a personal 

 idiosyncrasy, and of course it is easy to understand that mere 

 amplification of image without sufficient resolving power to justify 

 it is useless, but in spite of the generally prevailing opinion to the 

 contrary, personally I have found that on many delicate objects with 

 the majority of the picked objectives of modern construction in 

 my collection, a x 40 ocular gives the best possible view for the 

 recognition and detection of previously unknown detail, when the 

 corrections are perfect and large illuminating cones are employed. 

 I have fully convinced myself that some well stained bacteria, the 

 endoplasm of which clearly exhibits definite structural features 

 with the |- in. apo. and x 40 eye-piece, only show the barest in- 

 dications of the existence of such internal structure with the same 

 objective and x 12 ocular, even when you know that the structure 

 exists. The x 24 and x 27 oculars have proved more satisfactory, 

 and would probably be quite sufficient for the examination of any 

 already well known object. 



The following two instances of the practical utility of deep eye- 

 pieces may prove interesting : — 



1. With a very good recent Powell achromatic 1 in. of N.A. 

 0'28 and solid axial illuminating cone of N.A. 0*22, the flagella 

 of cholera bacilli stained by Loffler's method can be quite clearly 

 and easily demonstrated with the x 40 eye-piece. They are still 

 easy, but not so conspicuous or well displayed witli the x 24 ocular, 

 while with the x 12 eye-piece they are practically invisible, and 

 the bacilli themselves appear little more than mere dots. The 

 objective, in this case, although a very fine one, is only a semi- 

 apochromat and possessed of no excessive aperture, yet even with 

 it the X 12 ocular fails to reveal certain known features which can 

 be most distinctly seen when higher eye-pieces are applied. 



2. A large Podura scale possessing very clearly marked struo- 



