LIMITS OF OPTICAL CAPACITY OF THE MICROSCOPE. 413 



It is to be hoped that a more general agreement on the essential 

 parts of the theory of the microscope will soon prevail, and that the 

 exaggerated significance of certain matters too long discussed in our 

 journals, will fade to its proper vanishing point. 



The theoretical limits of optical capacity of the microscope. 



In ''Poggendorff's Annalen," for 1874, Prof. Helmholtz published 

 an article, of which the following is a translation. 



"Whether, and to what extent, the optical performance of the 

 microscope is capable of further improvement, is a question of the 

 greatest moment for many branches of natural history. Doubtless, 

 some progress, and notably through the revival of Amici's suggestion 

 of immersion lenses adopted and carried out with such success by 

 Hartnack, has been made, bat each onward step is slow and 

 faltering. "We have, it is clear, arrived now at a point at which 

 any trifling gain is efl'ected with a disproportionate effort of mental 

 as well as mechanical labour. And yet, so far as I can see, no one 

 has been able to give any reason why this should be, excepting the 

 common belief that the difficalty lies in overcoming the spherical 

 aberration of lenses so small and of such quick curvation as is needed 

 for objectives of very high magnifying power. It is not long since 

 Herr Listing, one of the most eminent aathorities on this subject, dis- 

 cussed, (Poggendorff's Ann. v. 136,) the means by which it might 

 be possible to obtain amplifications ranging from 25 to 50,000 

 diameters, whilst in actual practice the ordinary range of serviceable 

 amplification is at the present moment limited, to, from 400 to 800 

 diameters. Moreover the collective experience obtained by repeated 

 efforts of practical opticians has taught us that all high amplifi- 

 cations combined with good definition (i.e., sharp delineation,) are 

 obtainable only by instruments in which the objective admits a 

 cone of light of very large angular aperture from each point of the 

 object. 



"We have gradually arrived at that stage of improvement in the 

 construction of instruments in which rays of light whose direction 

 is nearly perpendicular to the axis of the instrument are passed into 



