338 ACCESSORY APPARATUS 



from the under surface of the cover-glass, and comes back through 

 the oil and the objective to the eye-piece and the eye ; they are, in 

 fact, all optically continuous, so that the upper surface of the cover- 

 glass has ceased to exist optically, the only reflexion being from its 

 inner surface. It is here, therefore, that the oil-immersion system 

 gives a new value to this illuminator, by this means enabling it to 

 utilise a larger aperture otherwise unavailing. 



When this illumination is employed, if the eye-piece be removed 

 and the back of the objective be examined, it will be seen that all 

 that portion of the back of the objective whose aperture exceeds 1*0 

 is brilliantly illuminated. This annulus represents, and is produced by, 

 the excess of aperture beyond the equivalent air angle of 180, of 

 which it is also a measure. The internal dark space is of the exact 

 diameter of that of a dry objective of the same focus, and is the 

 maximum space which it can itself utilise on a dry object by trans- 

 mitted light. 



By means of this instrument carefully used, some difficult tests 

 and lined objects have been resolved ; but its principal use at the 

 present day is for the examination of metals, and it is eminently 

 serviceable in determining whether any dry-mounted object is in 

 optical contact with the cover-glass or not. If it be not so it is in- 

 visible with the vertical illuminator. So also it is instructive to 

 examine the backs of objectives of various apertures with this mode 

 of illumination. A dry objective will be wholly without the bright 

 annulus, while an immersion of 1*1 N.A. will have a narrow annulus r 

 and that of 1*4 or 1'5 a broad and still broader one. In this way, 

 by practice, a fair approximation to the aperture of an objective may 

 be obtained. 



It is not the absolute size of the annulus, but the relation of the 

 size of the annulus to that of the whole back, that must be estimated. 

 Thus ^-th of N.A. 1'2 will have as broad an annulus as -j-^th of 

 1*4 N.A., but the diameter of the back of the Jth is, of course, much 

 larger than that of the iVth, and this involves the necessity for a 

 relative comparison. 



Appliances for the Practical Study of Living and other Objects 

 with the Microscope. Stage-forceps and Vice. For bringing under 

 the object-glass in different positions such small opaque objects as 

 can be conveniently held in a pair of forceps, the stage-forceps (fig. 

 285) supplied with most microscopes provide a ready means. These 

 are mounted by means of a joint upon a pin which fits into a hole 

 either in the corner of the stage iteelf or in the object-platform ; the 

 object is inserted by pressing the pin that projects from one of the 

 blades, whereby it is separated from the other ; and the blades close 

 again by their own elasticity, so as to retain the object when the 

 pressure is withdrawn. By sliding the wire stem which bears the 

 forceps through its socket, and by moving that socket vertically 

 upon its joint, and the joint horizontally upon the pin, the object 

 may be brought into the field precisely in the position required ; 

 and it may be turned round and round, so that all sides of it 

 may be examined, by simply giving a twisting movement to the 

 wire stem. The other extremity of the stem often bears a small 



