Barker — On Microscopic Illumination. 
9 
American microscopists are only very lately extending the principles of 
liis instrument. However, I believe tliat tliose means of illumination 
which are placed in the axis of the microscopic tube will ultimately 
be found to be the best, as they do not throw shadows on any part of 
the object. 
I have as yet practically examined only one form of this axial 
illumination, in connexion with the principle of immersion, and have 
made use of the glass paraboloid as the one best suited to the experiment. 
Wenham, to whom we owe so much in microscopic science, was the 
first to introduce the parabolic reflector, which afterwards assumed the 
shape of a truncated paraboloid of glass, to be placed beneath the 
stage, whereby parallel rays of light are concentrated from all azi- 
muths to a focus in which the object is placed ; the central rays are 
stopped off, so as not to allow useless light to flood the field of view ; and 
a cup-shaped cavity is made above, to allow the light to leave the para- 
boloid without deviation {see Plate I., fig. 2). It is to this apparatus 
that I have applied th-e immersion plan, and it seems to me to have re- 
moved almost ail its imperfections, and to place the microscopic object 
under circumstances similar to those under which objects submitted to 
unassisted vision are best seen. In the first place, in the old construc- 
tion, there is great loss of light {see Plate I., fig. 1), from the way in 
which the rays of light leave the paraboloid and strike the under surface 
of the slide ; and the most valuable rays (those most oblique) are lost in 
much greater proportion than others. Secondly, the light undergoes dis- 
persion, if the obj ect be in balsam or fluid — indeed, in any case — and the 
oblique rays cease to be achromatic. To obviate some of these imper- 
fections, Wenham has lately suggested the use of a small, deep plano- 
convex lens, to be cemented to the under surface of the slide {see Plate II., 
fig. 1) on which the object to be examined is placed; but this it is 
practically impossible to use in investigations of the ordinary kinds. 
By making, in the construction I would suggest, the top of the 
paraboloid flat {see Plate I., fig. 3), and introducing a film of water, or, 
better, a fluid of a deflective power as nearly as possible equal to that 
of glass, between it and the under surface of the slide, nearly all these 
imperfections will vanish ; for optical contact will then be made between 
the paraboloid and slide ; and also the film of water will act as a water 
joint, and allow free action to the stage movements, so that any part of 
the slide can be easily examined. The oblique rays are thus best econo- 
mized ; little dispersion takes place, if the object be in liquid or balsam, 
and there is sufficient brightness for all powers. Another advantage 
arises unexpectedly ; for if the focus of the paraboloid, oe made a little 
higher, or if a slide of extra thinness be used, the oblique rays will 
undergo total reflection from the upper surface of the covering glass, and 
besentdown on the object so as to illuminate it by reflected light, a deside- 
ratum not hitherto accomplished in a satisfactory manner {see Plate III., 
fig. 1) ; the light also does not strike the object glass, which is a great 
source of glare in many of the other forms of oblique illumination. 
I have placed on the table this illuminator, made by Mr Yeates,. 
R- I, A. PUOC. — VOL. I., SER. II,, SCIENCE. C 
