962 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Mr. E. M. Nelson * somewliat severely criticizes the instrument. 

 With it used dry, he can just get a dark field with an objective of 

 0-82 N.A., but the effect is far better with an objective of 0-8 N.A. 

 " Therefore, as far as dark fields are concerned, it does all that is 

 claimed for it. As regards the quality of the dark field, it fails, as 

 every other illuminator, which only gives an oblique pencil in one 

 azimuth, must fail. A diatom, to be shown critically on a dark ground, 

 must be illuminated all round ; one edge is always blurred when the 

 illumination is from one side only. One will say then, that, if it is 

 not good as a dark-ground illuminator, it must be a first-rate stri93 

 resolver. This, however, is not the case. For an instrument to be 

 a good stri^ resolver it must be capable of varying the obliquity of 

 the illuminating pencil. The strongest resolution is always obtained 

 just before the field gets dark. Of course I am aware that the 

 obliquity of the illuminating pencil may be varied to a small extent 

 by dodging with the mirror ; but that fidgety sort of business cannot 

 be compared with the certain method of a central and focused con- 

 denser and a slot cut to a known depth. As mine is mounted, you 

 can neither rotate the beam about the diatom, nor the diatom about 

 the beam." 



On this "F. E. M. S." saysf that it "is unquestionable that the 

 little device will do good service within the limits prescribed by its 

 aperture. Mr. Nelson should not taboo it for not possessing powers 

 beyond the scope of its designer. He would hardly consider it fair if 

 the ' Nelson ' Microscope-lamp were publicly condemned for not 

 being provided with all the luxurious movements of the ' Dallinger ' 

 lamp. The construction of an illuminating apparatus of special 

 convenience and efficiency is almost invariably a question of expense. 

 Greater outlay would convert the ' Nelson ' lamp into a ' Dallinger.' 

 The ' Nelson ' costs some five guineas ; and yet is there any feat of 

 microscopical illumination possible with it that could not be done 

 with a sixpenny paraffin lamp and brown paper diaphragms in the 

 hands of an expert — say, in the hands of Mr. Nelson himself? " 



Lord S. G. Osborne replies | to Mr. Nelson's criticism, and " still 

 confidently recommends the instrument to the very many observers 

 who have no substages, not for hypercritical study of diatoms, but as 

 giving most lovely pictures of some of Nature's most beautiful 

 work." 



Wallich's Condenser. — Dr. G. C. Wallich has patented an im- 

 proved condenser intended to obviate the difficulty which has hitherto 

 been experienced in adequately illuminating objects having consider- 

 able depth, and more especially when examined in the binocular 

 Microscope and with high-power objectives. It extends the range in 

 depth through which more or less transparent objects may be dis- 

 tinctly seen ; and, when used with the binocular, facilitates the 

 production and increase of true stereoscopic effect. The speciality 



* Eng. Mech., xl. (1884) p. 157. See also further remarks, p. 242. 

 t Ibid., p. 199, and see pp. 263-4 (1 fig.), 

 i Ibid., pp. 180-1. 



