214 ON THE THEOET OF THE MICEOSCOPE. 



the last refraction through the posterior surface of the objective, 

 and the several refractions taking place in the ocular, (field lens 

 and eye-piece) by which the image is re-formed at the distance of 

 clear vision with diverging visual angles. The first act answers 

 plainly to the f auction of an ordinary "magnifying glass;'* 

 while the second, taking all the changes comprised therein 

 together, answers as obviously to the functions of the telescope 

 (possessing only a small objective aperture) to which the virtual 

 image formed by the first process serves as " object."* 



This analysis of the collective action of a microscope system of 

 lenses is fully confirmed by the circumstance that the site of the 

 final refraction taking place in the objective, which gives to the 

 now parallel ray a converging direction towards the ocular, 

 is always to be found in the posterior focal plane of the objective. 

 An imaginary lens separated at this place from the objective 

 system, and possessing a focal length which answers to the 

 length of the tube of the microscope, would represent the ** object 

 glass " of a telescope whose eifective (visual) angular magnifying 

 power might be computed according to rule by estimating 

 the power of the ocular and the length of tube. The 

 equivalent focal length of the front portion of the objective, 

 here supposed to be performing the work of a simple magni- 

 fying glass, would remain as before (the same as) that of 

 the objective itself, and would give by known rule the means 

 of determining the visual angle under which the object before the 

 microscope must appear as virtual image, formed by parallel rays 

 at endless distances. This interlocking of objective and ocular 

 fanctions — presenting the combined effect of a magnifying glass 

 and that of a telescope — must be laid down as the most general 

 and correct characteristic of the principle upon which the com- 



* In illustration of tliis mode of analysing the action of the microscope, 

 it may be worth while to point to the fact that when a properly corrected 

 lens (of any chosen focal length) is centred in front of a telescope, the 

 action of the last becames that of a microscope » 



