MICROSCOPIC ILLUSTRATIONS. 28/ 



while (17) shews the contrast of the pencil of a triple 

 object-glass of nine-tenths of an inch focus, also of a 

 quarter of an inch of aperture. 



METHOD OF FINDING THE DOUBLE APLANATIC FOCI OF 

 OBJECT-GLASSES HAVING THEIR INTERNAL CURVES IN 

 CONTACT. 



For the discovery of these, it will be advisable to have 

 an engiscope constructed with a pull-out tube capable of 

 considerable contraction and elongation ; the rules already 

 laid down will suffice for their detection, with the help of 

 some extracts from the valuable paper of Mr. Lister in 

 the Phil. Trans, for 1830, p. 187. Mr. L. says, p. 195, 

 "in general an achromatic object-glass, of which the inner 

 surfaces are in contact, or nearly so, will have on one side 

 of it two foci in its axis, for the rays proceeding from which 

 it will be truly corrected at a moderate aperture ; that for 

 the space between these two points its spherical aberration 

 will be over corrected, and beyond them either way under 

 corrected." Again, p. 196, "the longer aplanatic focus 

 may be found when one of the plano-convex object-glasses 

 is placed in a microscope, by shortening the tube, if the 

 glass shews over correction ; if under correction, by length- 

 ening it, or by bringing the rays together, should they be 

 parallel or divergent, by a very small good telescope. 



"The shorter focus may be got at by sliding the glass 

 before another of sufficient length and large aperture that 

 is finely corrected, and bringing it forwards till it gives the 

 reflection of a bright point from a globule of quicksilver 

 sharp and free from mist, when the distance can be taken 

 between the glass and the object. 



