16 Rheinberg: Common Basis ofthe Theories of3Iicroscopic Vision. XIX, 1. 



slit adjus ted to say -^U^ mm width, ^ bnt liaving tbe objective slit 



Wide open. 



Place a piece of riiby or malacliite greeu glass between tlie 



source of light and condenser in order to 

 liave approximately monochromatic light. 

 For convenience sake we will call 

 tlie stage slit P, its Image P' and tlie 

 slot above the objective AB. P' will 

 now be well defiued witli perfectly sliarp 

 edges (fig. 15). Tbeu whilst looking at 

 it gradnally narrow down AB and as 

 soon as it has been reduced to about 

 2 millimetres, several separate briglit lines 

 will be observed very near tbe edge of 

 P' . Continuing the narrowing down of 

 AB tbe brigbt lines will gradnally become 

 fnrtber and fiirtber separated from P', 

 and at tbe same time P' Avill broaden 



14. 



ont and its edges instead of remainiug 

 sbarp will fade off from ligbt to dark 



(fig. 16 and 17).- Tbe separate lines 



S object; objective; C 

 plate in which objective is 

 fixed ; J) diffraction grating 

 or slots used above objec- 

 tive; iV double nose piece; 

 L objective for examining 

 size of Slots; T piece of (ö*' wbicb we will designate tbe first pair 



open tube. nearest to P', S and S' and tbe second 



pair U and ü') also widen out and merge 



from a certaiu amonnt of brigbtness at tbese centres to complete 



darkness. If we drop into tbe eye piece a micrometer disc ruled 



now be laid above the objective; trouble as to' having them in carriers 

 of any particular size is avoided, they are easily accessible, can be sliifted 

 about and instantaneously removed or exchanged. They can moreover be 

 examined in situ by means of another objective on the double nose piece of 

 the microscope. As this objective when turned aside, prevents the body 

 tube being brought right down on to the objective, a piece of piain open 

 tube is screwed into the other side of the double nose piece. 



1) The width of the slit can be determined without any trouble by 

 having a micrometer disc in the eyepiece ruled in divisions of Vio ^^^ 

 and dividing the number of divisions it Covers by the known magnifying 

 power of the objective at the distance in question. 



■-) Figures 15, 16, 17, 21, 22, 23, 27, 28 are good representations of 

 the original photographs. Some of the negatives required several ln.urs 

 exposure, and were in consequence very thin so that an attempt to re- 

 produce by photomechanical process proved unsuccessful and woodcuts 

 were resorted to. 



