1905.] on High Poiv&r Microscopy. 17 



;■ "; Now it has hitherto been taken for granted that the highest 

 resolving power would be developed by means of the self luminous 

 surface ; but quite recently Lord Rayleigh has made this assumption 

 the subject of mathematical investigation,* with the result that it 

 now appears that certain forms of regular illumination — light struc- 

 ture, as it may, perhaps, be termed — afford a background on which 

 very minute objects can be better displayed and more vigorously 

 delineated than upon the structureless field of a self-luminous area. 

 Lord Rayleigh's discussion of this topic has been purely mathe- 

 matical, and he must not be held responsible for the attempts 

 w^hich have been made to carry into practice the suggestion contained 

 in his paper. But such attempts have been made, and with a suffi- 

 cient promise of success to warrant a brief reference to the matter 

 in this place, although they are at present purely tentative, and 

 indeed, in the initial stages of a tentative effort. 



The exact practical problem, as it results from Lord Rayleigh's 

 theoretical conclusions, is to illuminate the stage of the microscope 

 with light, the phase of which shall change according to a rule of 

 variation from point to point, so that where a strong defining line 

 occurs in the object, it may be reinforced by the interference of the 

 light given off by adjacent parts of the luminous field. Such a con- 

 dition would not necessarily be satisfied by the borrowed light with 

 which a sheet of white paper, or say, the surface of the moon, shines. 

 But the method of mixing by reflection the emanations from a 

 primary light source is by no means the only mode in which a regular 

 structure can be given to a beam of light. Many other plans can 

 be followed, and one which is particularly susceptible of nice adjust- 

 ment and exact control is the employment for the illumination of 

 the field of the diffraction fringe formed upon the edge of a shadow. 

 It can be shown that in such a diffraction fringe the phase of the 

 light varies from point to point according to a law which results in 

 isophasal zones drawn parallel to the edge of the shadow ; and 

 experiment shows that when such zones are formed in the luminous 

 field of a microscope, and are arranged parallel to the outlines of an 

 object lying in that field, the outlines receive a notable accession 

 of density. Thus, if a test plate, say, for example, a Nobert or 

 Grayson ruling, be taken and viewed by means of an objective which 

 is barely powerful enough to resolve it under full illumination, it 

 may be seen strongly resolved if for the full light a suitably graduated 

 diffraction fringe be substituted. 



This experiment may be repeated upon a large scale by means of 

 the very simple appliances which are here upon the table. The 

 lantern is arranged so as to yield a slightly divergent beam of light. 

 The central pencil of this divergent beam may be regarded as con- 

 sisting of substantially parallel rays, like a beam of sunlight, and 



* See the Journal of the Royal Microscopical Society, 1903, p. 474. 



YOL. XVIIL (No. 99) c 



