The Helmholtz Theory of the Microscope. By J. W. Gordon. 401 



who lias ever attempted to brighten up the field of a Microscope 

 under excessive magnifications by recourse to sunlight will know 

 the peculiarly spotty appearance of the field which is so produced. 

 Some of the spots remain fast in the field of the instrument, others 

 move about with the eye. The first-mentioned take their rise in 

 spots and imperfections of polish upon the ocular, the last in the 

 cornea, lens and vitreous humour of the eye. This treatment has 

 long been known as a method of seeing entoptic objects and is in 

 fact very useful. But upon the whole, as the entoptic objects 

 become more visible, the delicate microscopic details become more 

 indistinct." 



Diffraction Phenomena. 



A third point of practical consequence is that these very 

 narrow parallel beams give rise to strong diffraction phenomena, 

 thereby causing the light of one point to spread over the image 

 of another, impairing its definition. This is, in Helmholtz' view, 



Fig. 89. 



the most important cause of faulty resolution in the Microscope. 

 His language is, "There is, in fact, a cause operative in the 

 compound Microscope which, under the given conditions, occasions 

 much more pronounced aberrations of the rays from their foci than 

 chromatic and spherical aberration, cause and which is most in- 

 fluential when the beam is narrow angled. That cause is the 

 diffraction of light." Accordingly, his paper, although it treats 

 incidentally of these other matters, is directed, as to its main 

 object, to an inquiry into the effects which are traceable to this 

 cause. Helmholtz prefaces his formal statement of the law by a 

 description of what is known as the Eamsden circle, and as this is 

 now a familiar object to microscopists, it may be sufficient here to 

 refer to this part of his paper in a very few words. 



