138 ACCESSORY APPARATUS. 



thin stage recently introduced into some of the best Microscopes 

 _(§§ 38, 39) possesses a great advantage over all whose thickness 

 is greater. On the other hand, it is when the rays are most 

 oblique, that the greatest advantage is gained by making them 

 fall upon the object from every side in succession ; and where 

 this cannot be accomplished (as in the case of ITachet's prism) by 

 the rotation of the illuminating appai-atus, the rotatory movement 

 must be given to the object. It is obvious that, for this purpose, 

 a revolving stage which keeps the object constantly in the field 

 (§ 37), is decidedly preferable to one which does not possess such 

 a movement ; but the means have not yet been found of obtain- 

 ing this advantage without some sacrifice of the other. 



61. Whenever the rays are directed with such obliquity, as 

 not to be received into the object-glass at all, but are suificiently 

 retained by the object, to render it (so to speak) self-luminous, 

 we have what is known as the hlach ground illumination ; to which 

 the attention of Microscopists generally was first drawn by the 

 Rev. J. B. Reade, in the year 1838, although it had been prac- 

 tised some time before, not only by the Author but by several 

 other observers. For low powers whose angular aperture is small, 

 and for such objects as do not require any more special pirovision, 

 a sufliciently good "black ground" illumination maybe obtained 

 by means of the concave mirror alone, especially when it is so 

 mounted as to be capable of a more than ordinary degree of ob- 

 liquity. In this manner it is often possible, not merely to bring 

 into view features of structure that might not otherwise be dis- 

 tinguishable, but to see bodies of extreme transparency (such, 

 for instance, as very minute Animalcules) that are not visible 

 when the field is flooded (so to speak) by direct light ; these pre- 

 senting the beautiful spectacle of phosphorescent points rapidly 

 sailing through a dark ocean. Where the mirror cannot be placed 

 in a position oblique enough to give this efiect, a black ground 

 illumination sufficiently good for many purposes may be ob- 

 tained by Mr. Reade's original method ; which consisted in dis- 

 pensing with the mirror altogether, and in placing the lamp and 

 ordinary conden sing-lens (§ 64) in such a position beneath and 

 to one side of the stage, as to throw upon the under side of the 

 object a pencil of rays too oblique to enter the object-glass after 

 passing through it. Another very simple mode, which answers 

 sufficiently well for low powers and for the larger objects which 

 these are fitted to view, consists in the substitution, for the 

 achromatic condenser, of a plano-convex lens of great convexity, 

 forming a large segment of its sphere, with a central stop to cut 

 ofi^ the direct rays ; for the rays passing through the marginal 

 portion of this Spotted Lens, being strongly refracted by its high 

 curvature, are made to converge at an angle too wide for their 

 entrance into an objective of moderate aperture, and thus the 

 field is left dark ; whilst all the liglit stopped by the object serves 

 (as it were) to give it a luminosity of its own. Neither of the 



