ON THE THEOKT OF THE MICROSCOPE. 257 



accessible to such objective is readily made; and it may be shown 

 that if ** resolving " power be not unfairly exalted at the cost of 

 the general excellence of the lens, there can be no question of 

 detail which a practised eye would not recognise with a good 

 amplification of from 4 to 500. Now according to the present 

 standard of technical constructive means, such an amplification 

 may be gained with an objective of 3 mm (^th English inch), 

 even if the attribute good be interpreted a little more strictly than 

 is often done. With immersion lense?, the physical limit of 

 "resolution," even where the angular aperture is the highest 

 attainable, does not extend so far that an amplification of from 7 

 to 800 will not be fully equal to it ; and this amplification would 

 be gained with ease with a well constructed objective of -}^ inch 

 focal length. It may be admitted that an amplification exceeding 

 the minimum here given, as theoretically necessary, might greatly 

 facilitate observation and render it more certain if the additional 

 amplification be as correct as can be possibly made, although it 

 would not occasion any new facts to be seen. Yet one can 

 scarcelv estimate the significance of this empty amplification far 

 beyond the limits stated, and I therefore come to the conclusion 

 that the scientific value of an objective whose focal length (if a 

 dry system) is much shorter than -^V inch, or if an immeraion 

 system, than ^V inch, is altogether problematical. 



The actual powers of the microscope (in the strict sense of 

 correct and useful power) are, in my opinion, exhausted at these 

 limits, so long, that is, as no circumstances of moment are brought 

 forward which change the bearing of present theory, There 

 exists no microscope in which there has been seen, or will be 

 seen, any structure which really exists in the object, and is 

 inherent in its nature, that a normal eye cannot recognise with a 

 sharply defining immersion lens magnifying 800 times. Heports 

 of extraordinary performances (especially from England) of 

 unusually high power (J^ inch?) are not of such a character as 

 to induce me to change my opinion and lead me into similar 

 •error, for the superiority of such lenses is said to have been 



