a2 ON INTERFERENCE IN THE MICROSCOPE. 



taken with the microscope wave lengths actually come into 

 operation which would have no effect on the visual organ. It is, 

 therefore, not without interest to tabulate the wave lengths of the 

 several rays with the limits of distinguishability which may be 

 therefrom deduced. As before mentioned, the values of these 

 limits with extreme oblique illumination would be expressed by 

 the corresponding half wave lengths. 



The numerals referred to as lines -3f and N are taken from the 

 communication of Draper, in Poggendorff^s Annalen, 1874, vol. i^i, 

 p. 2>2>1' The so-called chemical rays fall approximately with these 

 most refrangible rays. In microscope photography the conditions 

 are such, therefore, as to render the structure of an object photo- 

 graphed coarser than that seen through the microscope in the 

 proportion of 2 to 3. 



The proof that the delineation of fine structure is effected not by 

 dioptric means, but through interference of the diffracted rays, is of 

 the greatest significance for the interpretation of the microscope 

 image ; for, whilst the dioptric image as produced by homofocal 

 rays answers point for point to the object on the stage, and there- 

 fore affords a thoroughly safe inference respecting the actual 

 nature of that object (provided the stereometric effect of 

 what is seen on a flat field be correctly interpreted) the 

 interference images which arise by co-operation of diffraction 

 phenomena stand in no constant relation to the nature of the 

 object. Rows of points, for instance, deliver the same image as 

 actual striae 3 and when two such systems of striae cross each other 



