404 NOTICES OF THE MEETINGS [March 10, 
arm of a horizontal graduated circle, and is adjusted so that the 
point of a needle, made to coincide with the axis of motion of the 
movable arm, may be in focus and in the centre of the field of 
view. The other microscope, to which the object-glass to be 
examined is attached, is fixed, and so adjusted, that the point of 
the same needle may be in focus in the centre of its field. The 
eye-piece of the latter is then removed, and a cap with a very 
small aperture is substituted, close to which a lamp is placed. 
It is evident that the rays transmitted by the aperture will pursue 
the same course in reaching the point of the needle, as the visual 
rays from that point to the eye, but in a contrary direction, 
and being transmitted through the movable microscope, the eye 
will perceive an image of the bright spot of light throughout that 
angular space that represents the true aperture of the object glass 
examined. The applications of this instrument in the construction 
of object-glasses are too numerous to be here detailed: amongst 
the most obvious of which may be mentioned the ready means it 
presents of determining the nature, and measuring the amount of 
the aberration in any given optical combination. 
The important subject of Illumination was then so far considered 
as the short space of time allotted to the discourse would permit. 
It may be taken as an axiom that in the illumination of transparent 
objects, the amount of definition will depend on the accuracy with 
which the illuminating rays converge upon the several points of 
an object ; consequently the source of light and the field of view 
must be the conjugate foci of the illuminator, of which an achro- 
matic combination, similar to an object- glass, is the best form, and 
the common mirror usually employed is probably the worst, 
inasmuch as in a pencil of rays obliquely reflected at a spherical 
surface, no focal point exists. 
The first compound microscopes on record, as those of P. 
Bonnani, about 1697, which was placed horizontally, and that of 
J. Marshall in the beginning of the eighteenth century, which was 
vertical, were furnished with central condensers, but in later 
years the perfection of the illuminating apparatus has by no means 
kept pace with that of the ocular portion of the microscope, 
though scarcely of less importance, in attaining the utmost practi- 
cable perfection in the vision of microscopic objects. 
The advantages of employing an achromatic condenser were 
first pointed out by Dujardin, since which time an object-glass 
has been frequently, but inconveniently employed, and more 
recently achromatic illuminators have been constructed by most 
of our instrument-makers. 
Some years since Mr. Gillett was led by observation to appre- 
ciate the importance of controlling not merely the quantity of light, 
which may be effected by a diaphragm placed any where between 
the source of light and the object, but the angle of aperture of the 
