Critical Microscopy. By E. M. Nelson. 283 



should be fitted with a diaphragm, and there should be a substage 

 or tube for receiving the accessory apparatus, condenser, etc., 

 which may afterwards be added." This shows that in the writer's 

 opinion a substage-condenser is not regarded as a necessity, but 

 rather that it falls under the category of a polariscope, spot-lens, 

 or paraboloid. 



In those days no one ever thought of tackling diatom structure 

 without oblique light, and Grubb and Sollitt's methods* of a 

 swinging substage were revived. Microscopes were constantly 

 appearing with some new variety of swinging substage, but by 

 that time my mind with regard to the large axial cone was fully 

 made up, and a swinging substage, however well designed or 

 beautifully made, never became an allurement to me. 



The Amphiplcura pelhicida had just been resolved, and the 

 true number of the striae both seen and counted for the first time 

 by Messrs. Powell and Lealand with their new superstage oblique 

 illuminator,! a piece of apparatus still kept in my cabinet as a 

 curiosity (a device which has lately again come into notice in the 

 form of Dr. Siedentopfs method $ of side illumination). 



There were also the beautiful photographs by Colonel Woodward 

 both of Amphipleitra and Robert's bands taken by sunlight passed 

 through a blue screen and used oblique. § Just at this time too 

 there began to be published in the microsopical literature of this 

 country Professor Abbe's spectrum theory of microscopic vision, || 

 which showed that the resolution of periodic structures depended 

 upon the manufacture of a spectrum at one side of an object glass 

 by an oblique beam at the other. It was no wonder then that 

 with all this evidence in favour of oblique light, brought forward 

 by the leaders of microscopical science, I and my large axial cone 

 had a bad time of it. But repeated exhibitions of objects shown 

 by means of the large axial cone at the meetings both of this 

 Society and of the Quekett Microscopical Club bore fruit, so 

 by degrees less and less was seen of swinging substages and 

 oblique light illuminators, and gradually critical illumination was 

 adopted by those doing the highest class of microscopical work. 

 When the controversy was at its height in 1888 a photograph of 

 Pleurosigma angulatum taken by means of a small axial cone was 

 published If by Messrs. Zeiss, and in the next year Professor Abbe 



* Quart. Journ. Micr. Sci., iii. (1855) p. 87. See also this Journ., iii. (1880) 

 pp. 1055-80. (In the list on p. 1055 Sollitt's Microscope is omitted.) 



t Monthly Micr. Journ., i. (April 1869) pp. 315, 319. 



X See this Journal, 1903, p. 573. 



§ Monthly Micr. Journ., xviii. (1877) p. 61; Op. cit., i. (1878), p. 246, and 

 ii. (1879) p. 769, figs. 5, 6. 



i First notice, Monthly Micr. Journ., xii. (1874) p. 29. Full account, see this 

 Journal, 1881, pp. 303 r 60. (For Bibliography see 1879, p. 651.) 



^ Published as a leaflet, and widely distributed ; also in Catalog fur Mikro- 

 photographie, by Dr. Roderich Zeiss, 1888. 



