106 THE MICROSCOPE AND ITS REVELATIONS. 



every azimuth, it should be mounted, like the Disk-illuminator just de- 

 scribed, in an independent fitting-. 1 



102. The Amid Prism, which causes the rays to be at once reflected 

 by a plane surface and concentrated by lenticular surfaces, so as to answer 

 the purpose of Mirror and Condenser at the same time, is much approved 

 by many who have used it. Such a Prism may be either mounted on a 

 separate base, or attached to some part of the Microscope-stand. The 

 mounting shown in Fig. 77, is a very simple and convenient one; this 



consists in attaching the frame of the prism 

 Ero. w to a sliding bar, which works in dovetail 



grooves on the top of a cap that may be set 

 on the ' secondary body' beneath the stage; 

 the slide serves to regulate the distance of 

 the prism from the axis of the microscope, 

 and consequently the obliquity of the illum- 

 ination; whilst its distance beneath the stage 

 is adjusted by the rack-movement of cylindri- 

 cal fitting. In this manner, an illuminating 

 pencil of almost any degree of obliquity that 

 Prism. is permitted by the construction of the Stage 



may be readily obtained; but there is no pro- 

 vision for the correction of its aberations. In order to use this oblique 

 illumination to the greatest advantage, either the prism or the. object 

 should be made to rotate, thus causing the oblique rays to fall upon the 

 latter from every azimuth in succession, so as to bring out all its markings 

 ( 145). 



103. Black- Ground Illuminators. When the rays are directed with 

 such obliquity as not to be received into the Object-glass at all, but are 

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

 we have what is known as the Black-ground illumination. For low 

 powers whose angular aperture is small, and for such objects as do not 

 require any more special provision, a sufficiently good ' black-ground 

 illumination may be obtained by turning the concave Mirror as far as 

 possible out of the axis of the microscope, especially if it be so mounted 

 as to be capable of a more than ordinary degree of obliquity. In this 

 manner it is often possible, not merely to bring into view features of 

 structure that might not otherwise be distinguishable, but to see bodies of 

 extreme transparence (such, for instance, as very minute Animalcules) 

 that are not visible when the field is flooded (so to speak) by direct light; 

 these presenting the beautiful spectacle of phosphorescent points rapidly 

 sailing through a dark ocean. It is one of the great advantages of this 

 kind of illumination, that, as the light radiates from each part of the 

 object as its proper source, instead of merely passing tlirougli it from a 

 more remote source, its different parts are seen much more in their normal 

 relations to one another, and it acquires far more of the aspect of solidity. 

 The rationale of this is easily made apparent, by holding up a glass 

 vessel with a figured surface in front of a lamp or a window, at some dis- 

 tance from the eye, so that it is seen by transmitted light alone: for the 

 figures of its two surfaces are then so blended together, that unless their 

 form and distribution be previously known, it can scarcely be said with 

 -certainty which markings belong to either. If, on the other hand, an 

 opaque body be so placed behind the vessel that no rays are transmitted 



1 Ibid., Vol. i. (1878), p. 246. 



