1G4 J. RHEINBERG ON THE COLLECTED PAPERS OF 



In the researches as to the relations between aperture and 

 magnification, a great many important rules are given for the 

 selection of apertures and magnifications. They are expressed 

 concisely by the general rule, " Wide apertures for objectives of 

 short focus ; small and medium apertures for low- and medium- 

 power objectives." " Empty " over-magnified images are of no 

 use ; the employment of wide apertures in the case of low 

 magnifying powers is also to no purpose. Although such rules 

 now, perhaps, appear to be a matter of course, many a proposal 

 was made, especially in English journals, which, by means of 

 tables on the rational adjustment of aperture to magnification, 

 are relegated to their proper places. In this respect the closing 

 sentence is especially characteristic (p. 434) : " In my opinion, the 

 question here treated has a certain general importance for micro- 

 scopy. Of course, it does no harm if lens-systems of any type, 

 and for particular requirements, are produced, and in this respect 

 complete freedom must be allowed. On the other hand, as the 

 microscope has an important mission as the auxiliary of scientific 

 research, scientific microscopy is consequently fully justified in 

 claiming that improvements in the instrument should be always 

 directed primarily to making it as useful as possible for its chief 

 purpose, and that we should not ride any hobbies as regards the 

 optical construction of the microscope." 



The treatise on illumination by wide-angled cones of light gives 

 the explanation why the employment of these is serviceable for 

 deeply stained preparations (R. Koch's method), whereas for 

 uncoloured elements an obliteration of the whole image (Gesamt- 

 b'dd) is, in some cases, brought about. The coloured portions only 

 act by absorption, and the diffraction spectra produced by these 

 do not differ amongst themselves for different obliquities of 

 illumination ; whilst the uncoloured structures only operate 

 through different refraction and different retardation of the light 

 transmitted, and therefore produce unlike, separate images, the 

 mixture of which by wide cones of illumination causes the afore- 

 said obliteration of the image as a whole. 



The discussions as to the correct definition of magnification are 

 from the standpoint that the usual explanation of the magnifi- 

 cation of a lens, or a lens-system, is strange and irrational, in 

 which Abbe is certainly quite right. Instead of linear magnifica- 

 tion for a fixed distance, therefore, he proposed the introduction 



