2o8 Dr. a. Hodgkinson on 



e.g., a series of fine lines, of less than a certain degree of 

 fineness, is essentially dependent on their dififractive action,, 

 and seeing it admits of easy proof that, when the distances 

 between the centres of the lines constituting such structures 

 is less than half a wave-length of light, no diffraction can 

 occur even with light of any obliquity, it is evident that such 

 structure must be invisible under any microscopic power. 

 Since, moreover, such structure, so far as I am aware, 

 fails to produce any optical manifestation whatever, it can- 

 not be classed in the group we have termed ultra- 

 microscopic. To take one more example : — Structures of" 

 the same optical density and colour as their environment 

 yield no optical evidence of their existence, and belong,, 

 therefore, neither to microscopic nor ultra-microscopic 

 structure. A slip of crown glass, for example, is invisible- 

 in cedar-wood oil, and the same is true of other structures 

 of the same refractive index and colour. So far as 

 ordinary light is concerned such structures are non-existent. 

 On this fact, indeed, is founded the homogeneous immersion 

 system of lenses. 



It is thus evident that a class of structures exists- 

 which are wholly unsuited, from their physical nature 

 and that of light, for investigation by any known optical 

 method. Such structures might be aptly termed Hyper- 

 photic, since it seems unlikely they will ever be revealed 

 by the agency of light. With such a group, therefore, 

 the method of investigation we are at present considering 

 is in no way concerned, since an essential feature of 

 ultra-microscopic structures is that they so modify light 

 as to produce characteristic optical effects. It remains- 

 now to briefly refer to the relationship of such method 

 of ultra-microscopic examination to ordinary or unaided 

 microscopic investigation. In other words, can we attain, 

 results by its employment not to be attained b}- 

 means of the microscope alone? And, if so, are such 



