1854.] OF THE ROYAL INSTITUTION, 405 
illuminating pencil, which can be effected only by a diaphragm 
placed immediately behind the achromatic illuminating combi- 
nation. An elastic diaphragm, or artificial pupil as it might be 
called, was first proposed by Mr. Brooke, which was shewn to 
answer very well ina large model, and produced a remarkable 
semblance of vital contractility ; but mechanical difficulties inter- 
fered with its application, and the revolving diaphragm in the 
instrument, now well known as Gillett’s condenser, was sub- 
stituted.* 
When the rays of light converging on the field of view meet at 
a greater angle than that of the extreme rays that can enter the 
object-glass, the dark-ground illumination is produced, in which 
the objects are seen in strong lines of light on a dark ground ; 
this is best suited to objects having a well-marked outline, such as 
the spicula of sponge, or the shells of the polygastrica. This may 
be effected either by Wenham’s truncated parabolic reflector, or by 
a central opaque stop in Gillett’s condenser. 
The value of this kind of illumination in certain cases was shewn 
by its effect in rendering visible the persistent cell-walls in a speci- 
men of hard vegetable tissue, a section of a plum-stone, which 
could hardly be distinguished by the ordinary, or bright-ground 
illumination. 
A white cloud brightly illuminated by the sun has long been 
recognised as the best source of illumination, but as this is not 
often obtainable, the light of a lamp thrown upon a flat surface 
of plaister of Paris, or powdered carbonate of soda, has been used 
as a substitute. A flat surface of white enamel finely ground, but 
not polished, has been used with advantage by Mr. Gillett, as 
the surface can always be rendered perfectly clear by a little 
soap and water. By either of these means the glare resulting from 
throwing the unmodified light of a lamp on the object is com- 
pletely obviated. 
The effect of glare or diffused light in interfering with the vision 
of an object was illustrated by reference to an experiment of 
Professor Faraday’s, in which a screen of gauze partially blackened 
is held in front of a printed placard or diagram; the diffused 
light reflected from the white gauze considerably obscures the 
object, which is scarcely interfered with by the blackened portion. 
The influence of illumination upon definition was rendered very 
evident by placing the two halves of a fly’s tongue, similarly 
mounted, under two microscopes having precisely similar object- 
glasses and eye-pieces ; the one was carefully illuminated by an 
achromatic condenser, and artificial white cloud; the other, by the 
light of a similar lamp reflected from a concave mirror: the dif- 
* A description of this very useful apparatus has been recently published in 
the “ Elements of Natural Philosophy,” by Golding Bird and Charles Brooke. 
