268 Transactions of the Society. 



up the 0'5625 as against unity, wtich is the ideal maximum of 

 the dry lens. 



It is truly stated in the 'American Naturalist' that the Vertical 

 Illuminator " can only he successfully used in conjunction with an 

 objective of high balsam angle," and I hope the reason of this has 

 been rendered clear. 



In examining a dry object with reflected and transmitted light, 

 the optical phenomena are reversed : with reflected light we have 

 the bright annulus and dark centre ; but, with light transmitted 

 from below, we have the central portion of the lens traversed by 

 the illuminating pencil, which is, however, unable to penetrate the 

 dark circle by which it is surrounded. 



On objects mounted in balsam (or fluid) the Vertical Illuminator 

 fails, as far as resolution is concerned, and it is on these that the 

 various sub -stage immersion illuminators come into play, their 

 greater or less success depending exclusively on their ability to 

 induce the dioptric beam to penetrate the magic circle beyond the 

 limit of 180^, as unless the light can be seen to touch the margin 

 of the lens, its full power has not been developed ; hence it appears 

 that " vertical " illumination, in some form, is the only means by 

 which the whole of the resolving power of large-angled objectives 

 can be utihzed on dry slides, just as on balsam objects immersion 

 illuminators are indispensable. 



In the foregoing observations I have throughout spoken of the 

 bright riiig of light, and this may lead to the impression that the 

 whole of this ring is used, but this is not so ; in practice only a 

 small portion is employed, the greater part being shut off by 

 a suitable external diaphragm or stop, just as with immersion 

 illuminators in the sub-stage a part only of the marginal rays are 

 employed. 



This seems to suggest the substitution of a small totally re- 

 flecting prism for the parallel plate of glass, which, projecting 

 slightly over the margin of the lens, gives a much more brilliant 

 beam of light, but it has the disadvantage of, to a certain extent, 

 interfering with the diffraction spectra, and thus under some 

 circumstances, so diminishing the aperture of the glass, as to 

 interfere with its resolving power. 



The Vertical Illuminator was originally intended to be used 

 more as a Lieberkuhn for opaque illumination with medium powers, 

 its present use not having been foreseen. That it can be so used 

 with even greater efi'ect on balsamed objects now, when homo- 

 geneous immersion objectives are used, is obvious, because the light 

 passes as direct as it formerly did on to uncovered objects in air, 

 both the upper and under surfaces of the thin glass cover having been 

 optically abolished — but the number of balsam objects suitable for 

 opaque illumination with powers as high as an g, is very hmited. 



