644 SUMMARY OF CURRENT RESEARCHES RELATING TO 



convenient for this arrangement. On account of the increased working 

 height the author considers this method inferior to the condenser 

 adaptation. 



Glatton — Right and Wrong Way of using a " Magnifying Glass." 



1. The lens should bo held as far from the object as will afford a clear sharp 

 view of it. 



2. The eye should be at the same distance from the lens as the latter is from 

 the object. 



The advantage of the latter condition is very apparent when examining 

 portraits with a reading glass. I have frequently seen the glass held either close 

 to the eye or close to the paper, both of which are wrong — the latter absurdly 

 so, as no attempt is made to focus the object. 



English Mechanic, lxxxi. (1905) pp. 449-50. 



(2) Eye-pieces and Objectives. 



The Abbe Condenser used as an Objective.* — F. K. Studnicka, 

 after reminding his readers that the condenser of the Abbe illuminating 

 apparatus is an objective reversed, points out that by using it in the 

 latter way, with a proportionally stronger objective, a continuous series 

 of weak magnifications very useful for certain purposes may be obtained. 

 He considers that the cases in which such a method is likely to be 

 useful are : — 



1. That preparations can be quickly and simply explored, especially 

 when large (i.e. brain-sections). 



2. That the peculiarity of producing graduated magnifications 

 (according to working distance, etc.) will be welcome to an observer 

 who wishes to draw. 



3. That it may be made to answer the purpose of a preparation 

 Microscope, and so be economical to an observer. 



4. That with the help of the Abbe condenser and the plane mirror 

 an erect Microscope can be easily turned into a horizontal one, and be 

 used as an aquarium Microscope. 



5. That the peculiarity of neighbouring objects appearing reduced 

 or enlarged, or even in natural size, facilitates the drawing or copying 

 of objects — the usual drawing apparatus being now combined with the 

 Microscope. 



The author illustrates his methods with figures. 



Discrepancy between Diffraction Theory and Geometrical Optics 

 in Actual Instances of Telescope and Microscope Objectives.! — 

 K. Strehl has examined an improved achromatic Microscope-objective 

 made by A. Kerber, to test how far the lens performs what theory would 

 have predicted of it. The lens is of 4 mm. focal length and of • 6 N.A. 

 He is able to state the following discrepancies between diffraction theory 

 tind geometrical optics in this particular case : — 



1. The wave-surface of the colour C, which in and for itself has the 

 greatest spherical aberration, and, compared with the brightest colour 

 (550 /J./X), has the maximum chromatic aberration, approximates the 

 most closely to the ideal spherical surface of brightest colour. 



* Zeitschr. wiss. Mikrosk., xxi. (1904) pp. 432-9(3 figs.)- 

 t Central.-Zeit. f. Opt. u. Mech., xxv. (1904) p. 265. 



