XIX, 1. Rheinberg: Common Basis oftlie Theories ofMicroscopic Vision. 15 



wise woukl be , and a correspondingly ill defined and hazy general 



Image ensiies. 



This defect of a simple lens is overcome as mucli as possible 

 by siütable combinations — we need not enter iuto fiirtlier detail — 

 and tlie corrected lenses are terraed aplanatic. 



The aplanatism of lenses is a matter of degree. No combinations 

 are truly aplanatic for all colours , but good apocliromatics are 

 corrected in this respect for two colours in the bright part of the 

 spectrum. 



It remains but to be observed, tliat although a combinatiou may 

 be practically free from chromatic or spherical aberration Avhen the 

 plane of the conjugate foci is perpendicular to the axis AB of the 

 lens (fig. 12), or inclined to it in some particular direction, few or 

 perhaps we should say no lenses are equally 

 free from aberrations for all parts of their field. 



I will conclude this chapter by describing a 

 simple way in whicli the foregoing results may 

 be experimentally demonstrated with the micro- 

 scope. 



On the stage of the Instrument place au 

 adjustable narrow slit — the slit of a small 

 hand spectroscope does excellently , focus the 

 substage condenser outo the slit, and it is theu 

 practically equivalent to a luminous source. 



Between the nose piece of the microscope and the objective another 

 adjustable slit must be arranged — a short piece of tubing, with 

 a slot at botli sides through which the adjustable slit carrier is 

 pushed, will do.^ Now using a 25 mm objective, view the stage 



13. 



1) The firm Carl Zeiss supplies a suitable tube and carrier going 

 between nose piece and objective for about 5/ — (fig. 13). The carrier can be 

 rotated, which is a further advantage for the purpose for wliich it is pri- 

 marily intended, viz. : for shovving the Abbe diffraction phenomena. A set 

 of diapln-agms are supplied with the above, but no adjustal)le slit. The 

 lattcr can however easily be improvised. All microscopists who may be 

 interested in optica! phenomena, are advised to possess themselves of this 

 little piece of apparatus, wiiich is exceedingly useful for experiments of 

 the most varied kinds. — For an extended series of experiments with stops 

 above the objective, I have found the arrangement sliown in tigiire 14 to 

 answer well. Fix the objective in a plate of wood , placing this on the 

 stage of the microscope. Use the substage as the object carrier; horizontal 

 and vertical movements can readily be improvised. Grätings or slots can 



