ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



485 



from below, and the light, if too strons;, may be reduced by help of a 

 circular diaphragm with a central orifice. But if this diaphragm be 

 replaced by an opaque circular disk, out of whose margin a small circular 

 segment has been removed, as shown in fig. 80, then only 

 oblique rays will pass through the object. The result is that 

 the convergent action of the objective procures a lateral and 

 downward view of the object. The author obtained his best 

 effects with a Zeiss objective C, and compensation oculars 8-lH. 



C 



Zeiss Dark Ground Illumination by Stopping-ofF in the 

 Immersion Condenser.* — The firm of Carl Zeiss have de- 

 signed this apparatus for the convenient examination of living pj^ go. 

 bacteria, blood-tests, and serum-tests. The method adopted 

 is to insert a star-shaped disk into the diaphragm of the Abbe illu- 

 minating apparatus. The little central knob of this disk projects 

 upwards, and upon it is accurately placed a central disk of 24 mm. 

 diameter ; the diaphragm-holder is then clamped. During this process 

 the iris has been fully open. The entire fitting is now swung intO' 

 position. In the majority of cases arc-light illumination would not be 

 required. # 



Measurement of Light-absorption by means of Rotating Prisms 

 and Motionless Sectors.f — E. Brodhun describes, under the above 

 title, a species of photometer for the accurate determination of the loss 

 of intensity suffered by a beam of light in passing through a given sohd 

 or liquid substance. The apparatus is so contrived that the percentage 

 of light thus absorbed is read off on a graduated scale. 



Achromatic Illuminator.| — Percy Dunn describes an achi'omatic 

 illuminator for examining the surface of the eye, and other similar 

 purposes. The apparatus (fig. 81) consists of an achromatic lens. 



Fig. 81. 



of about I in. focus fixed in a metal clip, attached to which is a 

 lamp-holder for an electric light. The current is controlled by means 

 of a spring, by pressing which the light is turned on. The lamp is of 



* Special Catalogue, Jena, 1907 



t Zeit. f. Inst., xxvii. (1907) pp. 8-18 (8 figs.). 



X Lancet, 1907, i. p. 1718 (1 fig.). 



