426 Mr. Coddington on the Improvement 



doublet, than which I can hardly imagine anything more ex- 

 cellent as far as its use extends, its only defects being the very 

 small field of view, and the impracticability of applying it, 

 except to transparent objects, seen by transmitted light. 



Now the sphere has this advantage, that whereas it makes 

 a very good simple microscope, it is more peculiarly fitted for 

 the object-glass of a compound insti-ument, since it gives a per- 

 fectly distinct image of any required extent, and that, when 

 combined with a proper eye-piece, it may without difficulty be 

 employed for opaque objects. I have therefore endeavoured so 

 to combine it, and this has been my principal difficulty; for the 

 systems of len.ses which I have found employed for this pur- 

 pose, are so improperly constructed, that I have been forced to 

 have one made from original calculations, and get tools con- 

 .structed on purpose, which has necessarily been attended with 

 .some delay. 



The principle which I have adopted, after one or two pre- 

 vious trials, may be explained as follows. 



One great cause of the excellency of Huyghens's eye-piece, is 

 the condition which he himself designed to fulfil, namely, that 

 the bending of the pencil is equally divided between the two 

 lenses. Now this may be done for a microscope, thus: 



Let (Fig. 1.) be the center of the object-glass, 



F the place of the field-glass, 



E eye-gla.ss. 



Let 0F= 2 inches (for example) 



FE = 1 inch. 

 And let the focal length of the field-glass be 1 inch. 

 eye-glass ... ^ inch. 



These values satisfy the conditions of achromatism, and it will 

 easily be seen, that if V be the place where the pencil tends to 



