Obituary. 399 



Mr. Stephenson first heard of Professor Riddell's priority of the in- 

 vention, aud at once acknowledged it before the Society.* 



The papers on bichromatic vision and on a secondary polarising 

 prism have probably not received the attention they deserve. 



Professor Abbe's diffraction theory of microscopic vision was first 

 noticed in the Journal of this Society t in July 1874, in an editorial 

 paragraph on ' The Capability of the Microscope.' In the next year 

 Dr. H. E. Fripp, in the Proceedings of the Bristol Naturalists' 

 Society, J gave a translation of the original paper. This translation 

 was largely abstracted in the same year in the Journal of this 

 Society,§ and two years later, in January 1877, Mr. Stephenson read 

 a paper,|| again drawing the attention of the Society to this subject. 

 He was (and remained, so far as we know) a firm believer in the truth 

 of this theory and its necessary deductions. 



The next paper, on a Homogeneous Immersion Objective, was at 

 the time the cause of much dispute about priority. Sir D. Brewster 

 in 1837, and Professor Amici in 1844, experimented with water- and 

 oil-immersion lenses, the object of Brewster's experiments being to 

 get rid of dispersion, and the object of those of Amici to diminish 

 the excessive refraction at the plane surface of the front lens. Dr. 

 Koyston-Pigott, 8 in 1870, suggested turpentine If as an immersion 

 fluid, with a view to increase the aperture, because its refractive index 

 was similar to that of balsam. He also published a table of the 

 maximum apertures of dry, water-, and turpentine-immersion objec- 

 tives in terms of crown-glass or balsam angles. 



In 1871 Mr. E. B. Tolles 9 of Boston contributed two letters to 

 the Monthly Microscopical Journal, describing his experiments with 

 balsam immersion objectives and condensers, and in one letter twice 

 uses the word homogeneous in connection with this subject ; he also 

 says : " The case is totally and most obviously applicable to that of 

 the ordinary balsam-mounted Microscope object for an aperture far 

 above 82° of angular pencil actually traversing the object, and made 

 available in the view to the eye of the observer. For obtainment of 

 extremest angle, however, let one precaution be taken, viz., balsam be 

 used above the slide and balsam below." 



At this time (1871) a controversy about the aperture of objectives 

 was going on, and letters frequently appeared in the Journal referring 

 to experiments made with objectives immersed in balsam, and to the 

 measurement of their balsam angles. 



This controversy was still in progress in 1878, when Mr. Stephenson 

 read before the Society his paper on a wide-angled immersion objective 



* M.M.J., x. (1873) p. 41. t Op. cit., xii. (1874) p. 29. 



t Proc. Bristol Nat. Soc, n.s., i. (1875) p. 200. 



§ M.M.J., xiv. (1875) pp. 191 and 245. || Op. cit., xvii. (1877) p. 82, pi. 173. 



8 Op. cit., iv. (1870) pp. 26 and 140. 



% Turpentine was chosen as being homogeneous, qua dispersion, with the flint 

 concave of a doublet front, and, qua refractive index, with balsam. 



9 M.M.J., vi. (1871) pp. S4 and 214. 



