22 the president's address. 



position of the spectra, and not the detached short pieces which 

 form the sides of the hexagons ; for every side of a hexagon 

 is separated by twice its own length from the next side that 

 is in a straight line with it. In order that a filament of this 

 structure should break off and become separated, as in Mr. 

 Smith's specimen, it is necessary that the silex forming the 

 sides of the hexagons in the third direction should be weaker 

 than that forming the other two. 



We now pass on to the subject for the evening, and you will 

 probably agree with me that the one selected for your kind con- 

 sideration is the most important in the domain of microscopy 

 that has appeared since 1875, when the Abbe diffraction theory 

 of microscopic vision was first published in the "Monthly 

 Microscopical Journal." I, of course, allude to the physical 

 theory of microscopic vision written by my friend Mr. Lewis 

 Wright in the " English Mechanic " * during the months of 

 September, October, and November of last year. 



The questions at issue are very large, for if Mr. Wright's 

 conclusions are correct the table of the " Limit of Resolving 

 Power" in the " R. M. S. Journal " is incorrect, because it is 

 only valid for spurious images, and the true limit for micro- 

 scopic vision becomes reduced, as we shall see later on. But, 

 before proceeding, we must first consider the Abbe theory, 

 both as it first appeared, and also in its present position. In the 

 first instance we had a dioptric image for structure larger than 

 the -5-5V0 °^ an incn > an ^ after that a diffraction image. In 

 accounting for other pictures recourse was had to a double 

 theory, a dioptric one for one portion of the image and a diffrac- 

 tion one for another part. In simple words we were told that 

 a cataclysm took place in the physical phenomena of light at 

 a point denoted by the Y5V0 °^ an incri - And we we re further 

 taught that it was impossible to know anything of minute 

 structures unless certain impossible conditions were fulfilled, 

 such as the grasping of the entire diffracted fan. 



There was also the Eichhorn theory with its alleged predic- 

 tion of structure. This has been entirely refuted both prac- 

 tically and theoretically, and moreover it was shown to be quite 

 at variance with the fundamental laws of the diffraction theory. 

 The diffraction theory in its pristine condition was therefore 

 both incorrect and illogical. As it now stands, however, with 

 * " English Mechanic," Vol. lx. (1894), Nos. 1537-38-40-42-43-45-47. 



