16 



NOTE ON EXPERIMENTAL PROOF THAT THE 

 DOUBLING OF LINES IN THE ABBE EXPERIMENTS 

 IS NOT DUE TO THE DIAPHRAGMS ABOVE THE 

 OBJECTIVE. 



By Julius Rheinberg, F.R.M.S. 



(Bead March 17th, 1905.) 



The experiment which I have the honour to bring before your 

 notice is intended to convince any one that the duplication 

 of lines and other effects in the well-known experiments of Abbe, 

 which are to be found in all text-books on the microscope, are 

 due to the object itself, and not to the diaphragms placed in the 

 back focal plane of the objective. The latter is a fallacy which 

 is referred to and explained in Dippers Handbook on the Micro- 

 scope (Second Edition, 1882).* But in recent years the suggestion 

 has again been brought forward that the diaphragms above the 

 objective are the primary cause of the effect — a view which 

 appears to be in a great measure based on the fact that in the 



Fig. 1. 



usual experiments on duplication of lines, stops are used, in which 

 certain portions are blocked out, so that they have, in fact, two 

 or three apertures [Fig. 1 (1)]. Diaphragms like these, it is 

 supposed, would tend to cause a duplication of lines under any 

 circumstances — which in some degree is perfectly true. My 

 purpose, therefore, is to show you the same effect of the dupli- 

 cation of lines of a grating, using a diaphragm in the upper focal 

 plane which has one single aperture only [Fig. 1 (2)] ; and in 

 order to demonstrate that the single aperture cannot produce the 

 doubling in question (although I scarcely suppose this will be 

 suggested by any one), we shall use the same aperture on the 

 same grating, and produce the correct effect and the doubling 

 alternately by merely shifting its position a trifle. The large 

 and somewhat unfamiliar-looking instrument on which the effect 

 is shown is the Abbe Demonstration Microscope, which is so 



* Published at Brunswick by F. Vieweg & Sohn. 



