1834.] On Cutudioptric Miscroscopes. 289 



present attempt to supersede. For this purpose, I select the microscope 

 invented by Professor Amici of Modena, as justly deemed the most 

 refined and perfect now in use. The optical principle of this instru- 

 ment is represented in fig. 2, PI. XIX, andis thus described in Dr. Brew- 

 ster's elegant treatise upon optics : " He (professor Amici) made use of a 

 concave ellipsoidal reflector, whose focal distance was 2 T 4 5 inches. The 

 image is formed in the other focus of the ellipse, and this image is 

 magnified by a single or double eye-piece, eight inches from the reflector. 

 As it is impracticable to illuminate the object, m n, when situate as in the 

 figure, professor Amici placed it without the tube or below the line RN, 

 and introduced it into the speculum A B by reflection from a small 

 plane speculum placed between m n and A B, and having its diameter 

 about half that of A B." I have marked the requisite position of the 

 object m n to facilitate the reader's conception. 



The reader will note that there are two reflections in the objective 

 part of this instrument ; one of which is useful only as introducing the 

 rays proceeding from the object, and contributes in no respect to the 

 magnifying virtue of the microscope. He cannot fail to observe also, 

 that the great size of the plane forms a very serious impediment to the 

 rays of light proceeding from the conca\e metal to the eye-glass. By 

 an improvement of Dr. Goring's, the size of the plane has indeed been 

 reduced to ^rd the diameter of the great mirror. But even in this 

 improved state it continues so material an obstacle, besides having other 

 and greater disadvantages to be touched upon in the sequel, that one 

 cannot avoid wishing it removed if possible, by introducing and illumi- 

 nating the object itself in the axis of the tube at the focus of the con- 

 cave speculum. This improvement I have endeavoured to effect by a 

 very simple optical contrivance, which will be easily understood by re- 

 ference to figure 3, representing the form of the instrument adapted for 

 diaphanous objects. A B is the concave ellipsoid or mirror, and C is 

 a transparent object situated directly in its focus : D and E are the 

 illuminating apparatus ; D being a bull's-eye lens, and E a very 

 small plane diagonal mirror, so situated as to reflect parallel with the 

 axis of the tube rays of light coming from the lens. By this arrange- 

 ment, an image of the luminous aperture F, is formed in the focus of 

 the lens and just behind the transparent body C. It is now apparent 

 that but one reflection is sufficient to form an image of a microscopic 

 body in the conjugate focus of the speculum; and that the diagonal 

 plane being of almost evanescent dimensions, presents no obstacle 

 worth mentioning to the rays in their passage to the eye-glass. 



The mere loss of light however, attendant upon the Amician construc- 

 tion, is a disadvantage of very secondary importance ; for it is obvious 



