OBJECTIVES AND OCULARS 15 



(having long free working distance) and high oculars must be 

 adopted. The latter procedure is also indicated when employ- 

 ing dark ground illuminators "or ultra-condensers, namely, in- 

 crease the magnification by the ocular. 



Limit of Magnification. A consultation of the tables of 

 magnification given in the catalogues of the leading makers of 

 microscopes and microscope lenses will show that with the modern 

 compound microscope employed in the usual manner with stock 

 achromatic objectives and Huygenian oculars, a magnification 

 as high as 1500 to 2000 may be obtained, and that with stock 

 apochromatics and compensating eyepieces this may still further 

 be increased to 3000, the upper limit of listed combinations. 



Theoretically there is no limit to the magnification which 

 may be obtained. But this must not be confused with resolving 

 power which enables us to see things clearly and permits differ- 

 entiating one part or structure from another. Great magnifica- 

 tion avails us nothing if the image be blurred and irrecognizable. 

 A little thought will show that there must be a limit to the 

 resolving power practically available beyond which we cannot go. 



The shortest violet rays producing the effect of light upon the 

 average normal human eye may be assumed to have a wave 

 length of approximately X 4000 (or 0.4 /i) 1 . It has been shown 

 that under ordinary conditions the smallest particle which will 

 be visible as a black spot upon a light ground must have a diam- 

 eter equal to at least half this value (Helmholtz-Abbe). More- 

 over, a lens, owing to diffraction, yields as an image of a point, a 

 diffraction disk and not a point. The final image may be con- 

 sidered as consisting of a series of diffraction disks or patterns, 

 and if the distances between bright points are such as to cause an 

 overlapping of the resulting disks or their surrounding circles, 

 a blurring of the image must result. Thus we are limited, in 

 our attempt to see and study infinitely small particles, by the 

 sensitiveness of the human eye, on the one hand, which cannot 

 properly respond to the stimuli of very short wave lengths, and 

 to the fact, on the other hand, that no matter how great the mag- 



1 One micron, designated by the Greek letter /*, is equivalent to one-thousandth 

 of a millimeter (o.ooi mm.). 



