THE COMPOUND MICKOSCOPE. 73 



©f the Society of Arts, the author would refer those of his 

 readers who would wish to gain more information upon the 

 matter. Another eye-piece sometimes employed is the in- 

 vention of Kamsden ; it consists of two planoconvex lenses as 

 in that by Huyghens, but the field-glass is reversed, or its 

 plane smrface is placed farthest from the eye-glass ; this in- 

 strument, which wiU be again alluded to in the chapter on 

 micrometers, is chiefly used when it is required to measure 

 the magnified image of any object, hence it has been fre- 

 quently called the micrometer eye-piece, the divided glass 

 being placed immediately in front of the field-lens. When 

 this eye-piece is used, the image is formed in front of the 

 field-glass, and, consequently, the focal point of the eye-piece 

 is outside the field-glass; but in the Huyghenian form, the 

 image of the object is formed at the diaphragm between the 

 field and eye-glass; hence the former has been termed the 

 positive, and the latter the negative, eye-piece. 



The mirror generally consists of a frame of brass, in which 

 are set two silvered glasses, one concave the other plane, 

 which should not be less than two inches in diameter; the 

 former reflects the light in converging the latter in parallel 

 rays. For facility of adjustment, the frame carrying the 

 glasses is made to turn in every direction, by means of joints, 

 and in the best microscopes it is adapted to a tube on which it 

 can be slid either up or down, and so be approximated to the 

 under surface of the stage, in order that the rays reflected 

 from the concave surface may be brought into a focus or not 

 upon any given object on the stage. In some microscopes 

 the plane mirror is replaced by one made of plaster of Paris, 

 which reflects a soft white light, or by a prism of glass, the 

 invention of M. Dujardin. Mr. Varley has suggested a plan 

 of covering the plane mirror with pounded glass, or carbonate 

 of soda, by which means the light of a bright cloud opposite 

 the sun may be artificially imitated, and even the rays of the 

 sun itself may be reflected, and so produce a soft white light. 

 The different modes of using the mirror wiU be alluded to in 

 the chapter devoted to the illumination of microscopic objects, 

 where, also, will be described several other kinds of appa^ 



