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annular condenser, and by the assistance of Dr. Southby, who most 

 kindly constructed one in conformity with my suggestions, I have 

 been enabled partially to put my theoretical views to the test of 

 experiment. Messrs. Smith and Beck have also since constructed 

 me one of these instruments, both of which I purpose exhibiting this 

 evening. 



The annular condenser consists of a ring of glass (hence the name), 

 which, if cut across in a vertical direction, passing through the axis, 

 would exhibit at the two sections a plane more or less triangular, 

 consequently similar to a ring made of a triangular piece of metal, 

 so that one of the planes is at right angles to the axis. The other 

 two planes vary according to the desired obliquity of the light, but 

 always so that the upper one nearest to the axis shall be at right 

 angles to the emergent rays, as the lower one is to the incident ones. 

 This ring of glass is fitted into a cell of brass, which screws into the 

 ordinary fitting of the achromatic condenser instead of the lenses. 



By this arrangement all the rays fall at the same angle, and in 

 every azimuth of the circle ; the reflection from the only surface 

 required to reflect the light is very perfect, owing to the peculiar 

 property of the internal reflection of light in dense media, referred 

 to in a previous paper ; and lastly, there is comparatively but little 

 difficulty in the construction. 



I regret that want of time has prevented my being able to carry 

 out the principle as far as I could wish : "the only instrument I have 

 as yet been able to procure was not in my possession until late last 

 evening, and it causes the light to fall at an obliquity of no less than 

 sixty degrees, which is far too great for the larger number of objects. 

 It appears, however, admirably suited for examining some of the so- 

 called Infusoria when mounted dry, as also cuticular appendages in 

 a similar condition, thin sections of bone, &c. 



I omitted to mention that the width of the planes will be found 

 most convenient if about the tenth of an inch. 



