292 On Catadioptric Miscroscopes. [June, 



Having now so fully described the microscope for viewing transpa- 

 rent objects, I shall very briefly notice that intended for opaque ones. 

 It is represented in fig. 4, where A B is the great mirror, perforated 

 in the centre for the purpose of admitting a cone of light proceeding 

 from the lens D, and forming an image of a luminous aperture (not 

 represented in the figure for want of space) just in front of the micros- 

 copic object C, situated in the focus of the metal. The attempt to con- 

 struct this instrument was at first attended with a great deal of trouble, 

 my original intention being to introduce a parallel beam of light, con- 

 densed by a combination of lenses, through a very small bole pierced in 

 the centre of the metal. But for want of proper mechanical assistance 

 the attempt proved abortive. I accordingly altered the plan, and en- 

 larged the aperture to about 0.2, as in the figure, and admitted light 

 through a bull's-eye. E, is a stop, to arrest the rays that would other- 

 wise pass out to the eye-glass. It may be coloured according to the 

 ground that is best adapted for displaying the object under observation. 

 After enlarging so fully upon the former instrument, it is not neces- 

 sary to enter into much detail regarding the present one. All the ob- 

 servations upon the .optical principle of the one apply with equal force 

 to that of the other. I regret, however, that I had not an opportunity 

 of ascertaining as satisfactorily by experiment the performance of this 

 instrument, the perforated metal having been accidentally shattered to 

 fragments by a fall. Some trials however, upon ordinary objects in 

 its incomplete state, convinced me that this construction would perform 

 well. It exhibited the brilliant scales of a curculio in a very pleasing 

 manner. If upon the back of the stop E, a small silver cup be fixed 

 so as to be turned round occasionally, it will enable us to vary the light 

 by which objects are viewed ; and to examine them at once by radiated 

 and transmitted rays. 



Nothing I conceive can be more simple than the optical principle of 

 these instruments ; a single reflection, and a single refraction. And 

 what on the other hand can be more complex than achromatic re- 

 fractors with their triple, quadruple, quintuple, sextuple, and even triple- 

 triple object glasses? As any ordinary reflector may be very simply con- 

 verted into one upon the new construction for diaphanous objects, 

 by merely substituting a small diagonal metal for the Amician plane, I 

 am not without hopes that some naturalist in possession of a standard 

 instrument will do me the honour of giving the new principle a fair 

 trial. The result with an object metal wrought by a good artist will 

 be decisive. 



I have already extended this notice too far to enable me by this op- 

 portunity to communicate some speculations I intended upon several 



