MICROSCOPIC VISION. 143 



as the dodo. Allow me for the moment to separate two 

 parts of the Abbe theory, merely for the sake of illustra- 

 tion ; the two parts being physically connected together, 

 they must in the nature of things be indivisible, but their 

 supposititious separation will aid us in tracing the sequence 

 of events. 



The two parts are, of course, the aperture question and 

 the diffraction theory. The aperture question was the 

 subject of the great controversy that took place in the 

 Monthly Microscopical Journal^ and a very interesting 

 literature it is from a historical point of view. The dis- 

 putants were Dr. Pigott, Mr. Wenham, R. B. Tolles, Col. 

 Woodward, the Rev. S. L. Brakey, and others. The con- 

 troversy began with a paper by Dr. Pigott in the 31.31. J. ^ 

 vol. iv. p. 20 (July, 1870). Although the author in this 

 paper put forward his views in such an objectionable form 

 that they called forth just and severe criticism in their day, 

 nevertheless there were in them the germs of some of the 

 great developments that have since taken place. On page 

 26, for example, we have a small edition of the table of 

 corresponding angles now published on the cover of the 

 R.3I.S. Journal. The balsam or crown glass angles he 

 characteristically calls "nascent angles." This paper is 

 continued in the September No. (1870), vol. iv. p. 134, and 

 there we find a plate showing the advantages of the im- 

 mersion system. Fig. 1 shows rays traced from a radiant 

 in balsam through the cover glass into water and then into 

 the lens front ; the resultant longitudinal aberration is 

 depicted by the rays being traced backward to the axis in 

 the usual manner. This Dr. Pigott calls the compensating 

 action of " Hydro-spherical Aberration." Now, as the tracing 

 shows only the aberrations arising from the passage of the 

 rays through two plane surfaces, it is evident that the Doctor, 



