46 THE NORTHERN MICROSCOPIST. 


W. H. Davies, Esq., in the chair, a paper was read by Mr. H. T. 
Vivian, F.R.A.S., one of the vice-presidents, on “The Principles 
which Guide Opticians in the Construction of Achromatic Com- 
binations of Lenses,” in which Mr. Vivian explained the difficulties 
which had to be overcome from the imperfection of the images 
formed by single lenses, and the mode of ascertaining whether in 
any combination the imperfections had been removed. Mr. Vivian 
exhibited a microscope, having an objective which he had himself 
constructed, in which he had given the lenses that form which he 
considered would best remove the imperfections, and which showed 
objects with great distinctness of outline and freedom from colour. 
THE VERTICAL ILLUMINATOR.—At a meeting of the Royal 
Microscopical Society, held early last year, Mr. T. Powell begged 
to call attention to his exhibit of 4. pellucida dry on the undersurface 
of the cover-glass and without base-slide, illuminated by the 
Vertical Illuminator, and viewed by his new 1-12 oil immersion of 
142° balsam angle. It will be remembered that at the Scientific 
Evening a similar exhibit had been made, and it had been suggested 
that the illumination was really reflected upwards from the surfaces | 
of the base-slide. In order to meet that explanation of the illu- 
mination the object was now completely exposed on its underside 
—there was no glass surface beneath to reflect light. 
Mr. Crisp said that Mr. Powell’s exhibition touched upon a 
curious optical phenomenon, namely, whether or not this was 
strictly ‘‘opaque” illumination. Mr. Stephenson had suggested 
to him that A. pel/ucida was not itself sufficiently opague to scatter 
by ordinary reflection so bright a light as was seen in the micros- 
cope, and that inasmuch as the image was only seen by using an 
immersion objective of numerical aperture greater than 1.0, it may 
be illuminated by a part of the zone of rays transmitted by the 
objective of greater inclination than the “critical” angle—the 
diatom, being in intimate contact with the underside of the cover- 
glass, allowed these rays to pass into itself, and they could not 
wholly emerge at the under-surface of the diatom by reason of 
portions of the surface acting as total reflecting surfaces, and thus 
reflecting back through the diatom these extra oblique rays which 
the large aperture of the objective transmitted to the eye of the 
observer as a remarkably fine resolution of the dry diatom. In 
fact, he regarded the diatom thus mounted to be “immersed” (to 
a considerable extent) on its upper side, whilst its under-side was 
exposed to air. 
In this view the explanation of the illumination appeared to him 
rational: the “immersed” side of the diatom cancelled the total 
reflection, which could be seen to take place in the adjacent parts 
of the field (the image of the lamp-flame being plainly discernible), 
_—.s ee 

