ZOOLOGY AND BOTANY, MICROSCOPY, ETC. . r >21 



focus of a ground -glass plate ; a short distance behind this plate is the 

 Microscope that receives the light by means of a concave mirror. With 

 this illumination 800 magnifications of living bacteria can be obtained 

 with the same ease as with a good stained preparation. 



Resolution of Grayson's Bands.* — A student, after detailing some 

 resolutions of Grayson's bands, says : " The net results of these experi- 

 ments show that on a bright ground a certain size of illuminating cone 

 is required to develop the resolving power of any given objective, but an 

 increase in the cone beyond that certain size is always accompanied by a 

 falling off in resolving power. On a dark ground the case is somewhat 

 different ; with a ground just dark and no more, the highest resolving 

 power of the lens is not developed, but all objects just short of the 

 minimum resolvable are well seen. When light of greater obliquity is 

 employed, the lens attains its maximum resolving power, but the resolu- 

 tion of objects well within its grip is impaired." 



Doubling of Lines in the Abbe Experiments not due to the 

 Diaphragms above the Objective.!- — J. Rheinberg demonstrates this 

 by using a single-aperture diaphragm, which he places in the upper focal 

 plane. A coarse grating of about 100 lines to the inch (the widths of 

 lines and spaces being equal) is placed on the object stage, and by giving 

 a lateral movement of about T V inch to the diaphragm the effect of 

 single and doubled lines is alternately produced. 



Limit of Visibility of Isolated Elements in the Microscope. | — 

 K. Strehl makes some observations on this subject. 



Bright Spots on a Dark Ground. — He regards the speculations of 

 Siedentopf and Zsigmondy partly as hypothetical, partly as not free 

 from objection, and therefore attaches more importance to their results 

 as actually attained. With the most intense sunlight an illuminating 

 system of N.A. 0*3, and an observation system of N.A. 1/2, and 

 strong oculars, the least value they obtained for the edge of their cube- 

 shaped gold particles was 4 /jl/a = - 000004 mm. 



Dark Spots on a Bright Ground. — On the basis of the diffraction 

 theory, with N.A. 1 ■ 5, wave-length 500 //. //,, eye sensitiveness limit 

 5 p.c, and a completely aberration-free pencil, the author has demon- 

 strated the following limits of visibility : 



Self-luminous. Illuminated. 

 Smallest diameter of round dark apertures .. 48 nn 34 "5 mx 



Smallest breadth of straight dark slits . . .. 10*5 „ 2- 5 „ 



The comparison of both methods of observation is just as instructive 

 as the results are important in the investigation after ultra-microscopic 

 bacteria. 



Achromatisation of Approximately Monochromatic Interference 

 Fringes by a Highly Dispersive Medium, and the consequent In- 

 crease in the allowable Path-difference.§— R. W. Wood obtained 



* English Mechanic, lxxxi. (1905) p. 339. 



t Journ. Quekett Micr. Club (1903) p. 173 (2 figa.). 



t Central Zeit. f. Optik. u. Mech., xxvi. (1905) p. 117. 



§ Proc. Amer. Acal. Arts anl Sci.. xl. No IG (1905) pp. 59.J-G );J (8 figs.). 



