364 SUMMARY OF CURRENT RESEARCHES, ETC. 



rection-adjustment) of Mr. T. Powell, jun. The illumination by 

 the vertical illuminator is well known to be one of the severest tests 

 for the corrections of an objective ; and the lens, it may be said, bore 

 the test excellently. Its large aperture provided abundance of light, 

 so that the fourth or fifth eye-piece could be used with either kind of 

 illumination. 



It was pointed out by Mr. Stephenson at the meeting in March 

 that the illumination by the Vertical Illuminator had been erro- 

 neously supposed to be in this case " o2)ac[ue " ; whereas, in fact, the 

 light is totally reflected from the internal surface of the base of the 

 object. The proof of this is that no objective whose limit of aperture 

 does not Aipproximately) exceed twice the critical angle from glass 

 to air (82^) will illuminate the object ; the whole (or very nearly so) 

 of the light is transmitted through the object as it is not opaque 

 enough to reflect back by ordinary reflection sufficient light to pro- 

 duce an image of its surface. But objectives of greater aj)erture 

 provide an outer zone of rays beyond the critical angle ; these rays 

 cannot emerge at the base of the cover-glass of a dry mount adherent 

 thereto except where the object adheres; the rays then pass into the 

 object and suffer total reflection (to a great extent) at its base, and 

 thus provide extremely oblique reflected light, which is transmitted 

 ujiwards through the body of the object, and renders the surface 

 structure visible.* 



The late F. A. Nobert.— At the March meeting of the Society the 

 announcement was made of the death of Mr. F. A. Nobert, of Barth, 

 Pomerania, whose rulings of fine lines on glass have for many years 

 past been regarded as marvels of dexterity by the scientific world. 

 Mr. Nobert's fame is especially connected with the production of test- 

 plates for the Microscope, particularly the plate known as the 19-band 

 plate, on which successive bands of lines are ruled of increasing fine- 

 ness of division, from the rate of 1000 to the Paris line to 10,000 

 (equal approximately to 112,000 to the English inch). It was for- 

 merly Mr. Nobert's opinion that the last four bands of his 19-band 

 l)late would never be seen resolved in the Microscope. This opinion 

 he was constrained to withdraw after careful inspection of photo- 

 graphs of the whole series of bands by Dr. J. J. Woodward, of the 

 Army Medical Museum, Washington, U.S.A., from which an accurate 

 count of the lines actually ruled was made by Dr. Woodward, and 

 admitted by Mr. Nobert. Mr. Nobert then proceeded to make a new 

 plate of 20 bands of lines varying from 1000 to 20,000 to the Paris 

 line. The lines on the tenth baud in this latter plate corresponded 

 in fineness of division to the'lOtli baud of the former plate. The 

 microscopists of the future have therefore Nobert's legacy before 

 them to resolve the lines on the later test-plate. Mr. Nobert was 

 extremely reticent as to the method of producing his fine rulings, 

 and it is doubtful if he has communicated to any one the secret of 

 liis process of making and adjusting the ruling points. 



* See tl.e di&cussion at the March meeting, infra, p. 373. 



