402 NOTICES OF THE MEETINGS [March 10, 
WEEKLY EVENING MEETING, 
Friday, March 10. 
Right Hon. Baron Parke, Vice-President, in the Chair, 
Cuar.es Brooks, M.A., F.R.S. 
SURGEON TO THE WESTMINSTER HOSPITAL. 
On the Construction of the Compound Achromatic Microscope. 
Mr. Brooke stated his motive in giving the Lecture to be an 
observation frequently made, that many who are in possession of 
the best inicroscopes, either for the purpose of pursuing original 
investigations, or of seeking rational recreation in acquiring a know- 
ledge of the structure of natural objects, do not develop the full 
power of their instruments, from a want of sufficient acquaintance 
with the principles on which the detinition of objects depends. 
After briefly adverting to the ordinary phenomena of reflection, 
the Lecturer illustrated those of refraction by a movable diagram, 
which readily explained the total reflection of a ray of light in- 
cident on the common surface of two media at an angle greater 
than the critical angle, corresponding to which the angle of refrac- 
tion is 90°. 
The aberration of rays reflected or refracted at a spherical sur- 
face was then alluded to; and although the reflectors employed 
in microscopes may be rendered free from spherical aberration 
by giving them an elliptic, and those of telescopes, a parabolic 
form, there is no practicable method at present known of con- 
structing lenses otherwise than with spherical or plane surfaces : 
and from the difficulty of obtaining sufficiently perfect reflecting 
surfaces, and of preserving them when obtained, refracting micro- 
scopes are now almost universally employed. 
Chromatic dispersion was then mentioned, and the usual mode 
of producing achromatism by the combination of various kinds of 
glass, which differ in their dispersive power, was illustrated by a 
combination of three prisms. The construction of achromatic 
object-glasses was next explained, as well as the nature of the aber- 
ration produced by the presence or absence of a plate of thin glass 
covering the object, and the mode of correcting it in object-glasses 
of high power, by varying the distance of the anterior from the 
posterior combinations, as first extensively applied in practice by 
Mr. A. Ross, and fully detailed in his article on the Microscope, 
in the Penny Cyclopedia. 
The angle of aperture of object-glasses was then explained, and 
the power of those of large angular aperture in developing the 
