416 Dr. Goring on Achromatic Microscopes* 



as to allow it (by means of a ball and socket or other similar 

 arrangement) to fold down on one side, and leave the space 

 underneath open, as well as to permit the flame of a candle 

 to be applied immediately behind the stage for transparent 

 objects, or before it for opaque ones. This simple kind of 

 illumination, unassisted by any condensing lenses or mirrors, 

 is by far the most effectual, and the least liable to cause op- 

 tical deception of any I am acquainted with. As, however, 

 the object-glass and opaque objects are both frequently placed 

 in jeopardy by this arrangement, it will be necessary to em- 

 ploy a condensing lens occasional^, which had better be 

 attached to the body, that its spectrum may always travel 

 along with it. The condenser should be composed of two 

 glasses, of a large angle of aperture, and of the same focus, 

 to be screwed together at pleasure when candle-light is used 

 (which requires their focus to be only one-half of what it is 

 for day-light to give the maximum of illumination) ; for the 

 brightness of the spectrum depends upon the aperture the 

 condenser has relative to its acting focus. 



4th. It will be found very useful to have a kind of boot 

 to slide over the object end of the compound microscope, to 

 enable us to insert it directly into liquids of all kinds to some 

 depth, and examine objects contained therein without dis- 

 turbing them : a tube, somewhat conical at one extremity, 

 and having a piece of very thin and very accurately-figured 

 plate-glass burnished into it so as to be water-tight, will be 

 a very effective piece of apparatus for this end. Another 

 variety should also be constructed with a diagonal reflector, 

 to give a lateral and periscopic view of the contents of a 

 vessel. 



5th. The illuminating mirror should be oval* (if not of 

 very large dimensions), so that it may form a round image 

 in the visual pencil, equal in diameter to it when placed at an 



* Having a plain glass on one side, and a face of plaster of Paris at 

 the other, this latter surface, employed to reflect the rays of the sun, gives 

 a beautiful imitation of the light of a whole cloud, so much esteemed by 

 all microscophs for transparent objects. I do not approve of concave 

 mirrors ; a bull's-eye lens under the stage does tlieir v^ork better, in my 

 opinion. 



