ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 243 



case of the concave spectacle lens, and re-focus tliesubstage condenser on 

 the Nelson-lens iris. 



2. With the 2 in. -objective the arrangement of the light is the same 

 (method No. 2), except that the Beck immersion-condenser, with front 

 removed (0'25 N.A.), is used instead of the dry parachromatic condenser. 

 With very low-power lenses of the type of the planar of Zeiss and the 

 micro-summar of Leitz, which are used without an ocular, another 

 method must be adopted, which we may call — - 



Illuminating System Xo. 3. — Here we use as the substage condenser 

 the macro-illuminator of Messrs. Watson. With lamp and Nelson lens 

 giving parallel rays, as in System 2, we — 



1. Focus the objective on the object by projecting the image on a 

 white card. 



2. Focus the S.S. condenser so as to show on the card a sharp image 

 of the Nelson-lens iris. 



3. Adjust the Nelson lens so as to show on the card a fairly sharp 

 image of the filament running exactly across the centre of the image of 

 the Nelson iris. 



■4. Then adjust Zeiss collecting-lens with iris on stand (4 in. focus), 

 at about 9|^ in. behind the stage of the Microscope, centre and adjust it. 



5. Focus the S.S. condenser so as to show a sharp image cf the Zeiss 

 collecting-lens iris. 



The field, with the exception of a narrow reddish fringe at the 

 edges, is well illuminated, and this fringe can be eliminated by slightly 

 closing the iris of the Zeiss collector. The full aperture of the objective 

 is not, however, completely illuminated, so that for absolute perfection 

 this method requires some modification. 



(5) Microscopical Optics and Manipulation. 



Theory of the Image-formation in the Microscope.* — H. Erfle 

 reviews a book lately written by 0. Lummer and F. Reiche, and entitled 



E. Abbe, Die Lehre von der Bildeutstehung im Mikroskop. ] One of 

 these collaborators, 0. Lummer, was an auditor of Abbe's lecture in the 

 winter of 1S87 when he propounded his theory. In association with 



F. Reiche he has extended and deepened Abbe's theory. While Abbe 

 deduced his expression for the light-action in the secondary image from 

 the Fresnel-Huyghens principle, the authors have made use of Kirchhoflf's 

 principle in combination with the equations of Maxwell's Theory of Light. 

 It appears that by this second method Aljbe's expression can be obtained 

 in the case of limitation of the light to small convergence angles in the 

 image plane. 



The book is divided into four chapters, preceded by a short introduc- 

 tion dealing with geometrical optics based on the wave theory. 



The first chapter deals with construction of the ray refracted through 

 a spherical surface ; image-formation of an axial point ; image-formation 

 through a centred system of refractive spherical surfaces ; Abbe's image- 

 equations ; image-formation by means of wide-angled beams. 



* Zeitschr. f. Instrumeutenk., xxxi. (1911) pp. 358-60. 



t Braunschweig : F. Vieweg und Sobn (1910) xii. 108 pp., 57 pp. and por- 

 trait of Ernst Abbe. 



