Theories of Microscopical Vision. By A. E. Conrady. 611 



•denser, this assumption would indeed be justified, for each point in 

 the object would send out the regular spherical waves concentrated 

 upon it by the condenser and derived from the conjugate self- 

 luminous point ; and, as the light from each point in the object — 

 being derived from a separate and independently vibrating point 

 in the source of light — would also be independent from, and in- 

 capable of interference with, that from any other point in the ob- 

 ject, the necessary consequence would be that each point in the 

 object formed its own image in the form of a spurious disc, quite 

 independently of adjacent points. 



But unfortunately it is quite impossible to strictly realise this 

 ideal state of illumination, and I shall show that even the best 

 conditions to be met with in actual practice depart so far from the 

 ideal that it is quite out of the question to adhere to the belief 

 that the object could behave like a self-luminous one. 



The following are the principal reasons for this statement : — 



1. Even assuming that the whole illuminating arrangements 

 were absolutely perfect, we should still have to reckon with that 

 spreading of the light concentrated by the condenser which is 

 caused by diffraction, for the condenser, like any other optical 

 instrument, would render each point in the source of light as a 

 spurious disc surrounded by diffraction rings ; and, as the former 

 would be at least of the same order of magnitude as the detail 

 resolvable by the Microscope of which that condenser formed part, 

 while the diffraction rings would cover several times that area, we 

 should even then have neighbouring points in the object receiving- 

 light from the same source, and therefore the possibility of inter- 

 ference phenomena between the light from such neighbouring 

 points. 



2. But the optical perfection assumed is probably never realised, 

 for it implies : — 



(a) Perfect freedom from spherical and other aberration in the 

 condenser, whilst in reality all condensers, when used with any- 

 thing approaching full aperture, show considerable residuals of 

 aberration, or, at best, easily recognisable spherical zones ; hence 

 the spurious discs produced by actual condensers are very much 

 larger than those assumed above. 



(b) Optical perfection of other media intervening between the 

 source of light and the object ; we should therefore have to have 

 optically worked slips of optical glass instead of the usual ones 

 of badly annealed common glass. 



(c) Perfect homogeneity of the object. In the majority of 

 cases the object itself would also take a share in spoiling the 

 optical perfection aimed at, for very few objects would be so thin 

 and uniform as to act like a homogeneous plate. 



3. The greatest source of diffusion of light is, however, formed 

 by the usual system of obtaining "critical light." For visual 



