370 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Next, a diatom, Ticeratium favus, was photographed with a ^-in.- 

 objective. With a visual focusing two photographs at different distances 

 were taken ; these were similar to one another and to what was seen. 

 The same diatom then had a slightly different visual focusing, with the 

 same result as before in the two pictures. Finally, two diatoms with 

 fine markings were focused visually and at two positions ; the markings 

 came out clearly in the photograph. The object of showing the results 

 was not to make out that the emergent pencil was ever actually, or 

 always approximately, a single line of light, but to intimate how often 

 the author had found, when projecting from an optical eye-piece, that no 

 change can be detected in the definition of the image as the screen of 

 the camera is moved. If a camera-lucida is placed on the eye-piece, the 

 image of a stage-micrometer can be thrown on a scale at 10 in. distance 

 or at 40 in. distance. The parallel rays emerging from the eye-piece 

 give the image of a point along a direction, at no definite position. 

 The image can be imagined at 40 in. distance as easily as 10 in." 



Mr. Croft also showed some photographs taken from sections of the 

 human eye ; he indicated that a divergent pencil from a small aperture 

 or from a convex reflecting surface of large curvature will give the 

 Purkinje figures as bright radiating lines, whereas the usual method of 

 sending light through the side of the sclerotic gives them as shadows. 



Brass, A.— Grnndesetze der Optik. 



[This treatise is continued and concluded.] 



Central. Zeit. Opt. Mech., xxvi. Nos. 20-4 

 (Oct. 15 to Dec. 15, 1905). 



Kerber, A. — Zur Theorie der schiefer Biischel (Zweiter Beitrag). 



Zeit. f. Instrumentenk., xxv. (1905) pp. 342-3. 



Meyer — Das Ultramikroskop. Band i., No. 1. 



Prytz, K. — Mikroskopische Bestimmung der Lage einer spiegelnden Flache. 



[The principle of this method is optical contact. It was noticed in this 

 Journal for 1905, p. 756.] 



Tom. cit., pp. 386-7. See also Ann. d. Physik, xvi. (1905) p. 735.. 



W il sing, J. — Ueber die Zweckmassigste Wahl der Strahlen gleicher Breunweite 

 bei Achromatischen Objektiven. 



[Discusses the theory of complete achromatism, and describes several kinds- 

 of glass whose constants would lend themselves to such a result.] 



Tom. cit., pp. 41-8. 



(6) Miscellaneous. 



Quekett Microscopical Club. — At the 430th ordinary meeting of 

 the Club, held on April 20, Mr. F. P. Smith brought forward two papers 

 — " On the Spiders of the Diplocephalus Group " and " A Catalogue of 

 the Literature dealing with Erigonine Spiders." A paper on " Stereo- 

 photomicrography " was read by Mr. H. Taverner, F.R.M.S., and a paper 

 on the same subject which had been communicated by Mr. W. P. 

 Dollman, of Adelaide. Mr. J. Rheinberg, F.R.M.S., gave a resume of 

 a long and technical paper on " Stereoscopic Effect and a suggested 

 improvement in the Binocular Microscope." 



At the 431st ordinary meeting, held on May 18, Mr. A. E. Smith 

 gave an account of the three methods he employed in stereo-photo- 

 micrography, and Mr. D. J. Scourfield, F.R.M.S., greatly interested the 



