Nature and Art, June 1, 18G7.] 
THE MICROSCOPE. 
183 
Bocket microscope lamp (fig. 8). This is manu- 
factured by Mr. Collins, and is a companion to the 
microscope which every microscopist must find of 
benefit. It consists of a firm circular brass foot, 
from which rises a solid rod which bears the lamp, 
condenser, and reflector. The lamp is one of the 
paraffin kind, but is of such construction that danger 
of explosion is avoided. The reflector and bull’s- 
eye lens, as well as the lamp, are moveable up and 
down upon the vertical rod ; and are so arranged 
that when placed in proper position with regard to 
each other, the whole three (lamp, reflector, and 
condenser) may be elevated or lowered without 
much re-adjustment — an immense saving of time 
for the worker. The illustration will explain the 
details. 
It is usual, when examining transparent bodies, 
to place between the mirror and the object a com- 
bination of lenses calculated to throw a very bright 
stream of light upon the object. This combination 
is termed an achromatic (non-colouring) condenser, 
and hitherto it has been nothing more nor less than 
an inverted object glass, a very expensive appli- 
ance. Lately, however, an amateur microscopist 
(we believe one of our constabulary), Mr. Webster, 
devised an arrangement of lenses which not only 
supplies the place of the old condenser, but give 
also what is known as dark-ground illumination 
pierced with a number of apertures of different 
sizes, and its use is to cut off superfluous light 
so as to give an object the exact amount of 
illumination it requires and no more. But it always 
happens that a slightly smaller or greater quantity 
of light than that which the aperture of the 
diaphragm supplies is required. To meet this want, 
Mr. Collins constructed the apparatus represented 
in the adjoining illustration. Beneath a cap of brass, 
which is placed below the stage, are arranged four 
moveable shutters connected with a level’, which in 
its turn is worked by a milled head. By turning this 
latter the shutters may be opened or closed to any 
extent — the aperture always retaining its original 
form — and thus the precise quantity of light re- 
quired is allowed to fall upon the object. 
The illumination of opaque objects is generally 
conducted with what is known as a bull’s-eye con- 
denser. This is simply a very large magnifying 
lens which, when placed between the object and a 
lamp flame, causes a bright spot of light to fall 
upon the latter. In the case of very minute ob- 
jects, however, viewed Avith high powers, the bull’s 
eye cannot be used, and until lately it was impos- 
sible to examine very minute bodies as opaque 
objects. The obstacles in the way were removed by 
a contrivance invented by Mr. Smith, an American 
microscopist, and which is now manufactured by 
Fig. o. 
(fig. 9). It consists of a double concave lens 
cemented to “ a very deep crossed lens ” fixed in a 
brass tube or body which fits beneath the stage of 
the microscope. A circular disk provided with 
curvilinear apertures and “ stops ” is also attached 
to it, and when used displays the object in white 
upon a black ground ; 
thus giving to such 
structures as the shells 
of the Foraminifera 
an appearance of great 
beauty. This is the 
W ebster condenser as 
first introduced ; but 
it has been much im- 
proved by the addition 
of Mr. Collins’ gradua- 
ting diaphragm , which 
we now venture to de- 
scribe (fig. 10). The 
ordinary diaphragm supplied with microscopes 
is a circular plate of bronzed or black metal 
Messrs. Powell and Lealand, of Euston Boad (figs. 
11 and 12). Mr. Smith made the object-glass its 
own illuminator by piercing a small hole in the 
upper part of it, and fixing opposite to this, and at 
a certain angle, a disc of glass; a lamp being placed 
near the aperture the light falls on the disc of glass, 
as shown in the figure, and is reflected by it upon 
the object; the light then coming back from the 
object, and traversing the disc, reaches the ob- 
servei-’s eye. The microscopist sometimes finds 
that, whilst there is only sufficient light to illu- 
mine his object satisfactorily, there is too much 
light around the object — the illuminated field of 
Anew, in other words, is too glaring. To obviate 
this, Mr. H. J. Slack has recently devised a plan 
which has been carried out effectually. He intro- 
duces a sort of shutter into the eyepiece. By 
Avorking this lie can so contract the field of view 
that it merely incloses the object. Thus, Avliilst 
perfect illumination is obtained, the eye is protected 
from the injurious influence of a brilliant stream of 
light. 
