ILLUMINATION. 103 



where 8 represents half the angle of aperture, and / the posterior 

 focal length of the Microscope. If B = 180, then d = 2/, or if, 

 instead of/, we substitute the magnifying power m for a range of 



250 



vision of 250 mm., 1 d = 2.- . With a combined amplifica- 



m 



tion of m = 1,000; d will equal '5 mm. ; and if m = 5,000, d = 

 '1 mm. The diffractions! action in this case may be exemplified by 

 making an aperture of about *1 mm. with a needle in a sheet of 

 tinfoil, and viewing through this diaphragm a wire-gauze held near 

 a candle-name ; the contour of the object will then appear indis- 

 tinct through the influence of the diffraction phenomena. 



This one fact suffices to show that every attempt at an 

 unlimited increase of the combined amplification, which many 

 opticians have aimed at in modern times, must be fruitless. The 

 diffraction effect is so great, with a magnification of even 3,000 

 linear, that all hopes of attaining higher figures without a corre- 

 sponding sacrifice of distinctness must be abandoned. In reality, 

 the amplification in the strict sense of the word is reduced to a 

 much smaller figure, on account of the conditions of the formation 

 of the structure-image, which will be discussed later on. 



XII. 

 ILLUMINATION. 



IT is not our intention to describe all the different illuminating 

 devices issued by the opticians; this information will be 

 found in extcnso in the well-known micrographical works of Mohl, 

 Halting, and others. It appears to us more important to elucidate 

 the generally very vague ideas prevalent regarding the influence of 



p* - f P* f 



1 We know that m = ; therefore / = - , or, since / is very small 



n* 250 



compared with *./= = 



m m 



