98 THE AMBKICAN MONTHLY [June 



whicli these remarks originate, is that in several ways 

 additional light is thrown upon that question. The 

 general conclusion at which I have arrived stated briefly 

 as before, is that the trustworthiness of a microscopic 

 image is in proportion as the object approximates to a 

 self-luminous conditio7i, and diminishes in proportion as 

 it is or has to be (for it may have to be) examined by 

 plane-wave illumination. This view is of most funda- 

 mental and practical importance to microscopy and mi- 

 croscopic optics. 



10. Supposing the "spectrum " theory to be true, as a 

 full representation, it was demonstrated that "micro- 

 scopic vision is sui generis. 



11. Another fundamental objection to the competence 

 of the theory as a general one, is found in the fact that 

 the character of a grating may be such, that its spectra 

 cannot give a proper image. 



12. The object may conceivably be self-luminous ; in 

 which case there will be no spectra, and the waves 

 emitted from diiFerent points of the object will be quite 

 heterogeneous, and in no permanent phase-relations. 

 Yet an image must be possible, and can in that case be 

 only analysed according to the Airy method. We can 

 only employ a really self-luminous object in experiments 

 with low powers of the microscope — perhaps up to an 

 inch. But even the results with such a power are deci- 

 sive of the real question ; and with high powers we can 

 more or less approximate to this kind of luminosity in 

 several ways. 



Thus, even a wide cone from the condenser approxi- 

 mates to it. Lord Rayleigh has shown how and why 

 this kind of illumination must introduce a large amount 

 of heterogeneity into the rays proceeding from the object, 

 and concludes " that the function of the condenser in mi- 

 croscopic practice is to cause the object to behave, at any 

 rate in some degree, as if it were self-luminous, and thus 



