ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 421 



MICROSCOPY. 



A. Instruments, Accessories, etc.* 

 (3) Illuminating' and other Apparatus. 



Biprism for the Greenoug-h Microscope.f — E, M. Nelson writes 

 that while the Greenough binocular works well with unmounted objects 

 seen by reflected light and objects viewed upon a dark ground with a 

 spot lens or condenser and stop, yet with objects seen with transmitted 

 light there is a difficulty in illuminating both tubes ; for obviously if 

 the mirror were placed so that hght was reflected in one tube, the mirror 

 would not be in a proper position for reflecting light into the other. 

 The difficulty was overcome by means of a biprism and was found to 

 answer the purpose perfectly. The prism, made by C. Baker, is mounted 

 upon a circular plate of glass and drops into the stop-holder at the back 

 of the condenser fitting ; it can be used with or without the condenser 

 or with or without the stop. 



Zeiss Abbe Refractometer.:]: — A communication has reached 

 " Nature " from Bellingham and Stanley, Ltd., in which attention is 

 directed to an interesting point in connexion with the design of the 

 Zeiss Abbe refractometer. It has Iteen observed recently by -users of the 

 instrument that, owing to want of illumination, measurements cannot 

 be made for liquids having a refractive index greater than 1'52. It is 

 plainly stated in the Zeiss catalogue that the Abbe refractometer may be 

 used for the measurement of refractive indices from 1 • ;;0 to 1*7, and 

 that the liquid to be examined is enclosed between two prisms of flint 

 glass. In the instrument as actually constructed it appears that a crown 

 glass pirism of low refractive index (Ni,= 1'52) lias been substituted for 

 the dense flint prism (Ni, = l-75) used at first as the lower or illuminat- 

 ing prism. The contact surface of this prism is left unpolished, so as to 

 scatter the light entering the liquid film. The process of wiping the 

 surface to remove the liquid which has been examined results in the 

 removal of the thin sharp walls left by the abrasive, and the surface 

 approximates to a polished face. When this is the case very little light 

 can fall on the contact surface of the liquid and the upper prism at 

 angles greater than the critical angle unless the lower prism has a 

 refractive index greater than that of the liquid under test ; for it is only 

 when this condition is satisfied that light entering the liquid is bent 

 away from the normal. Several such instruments have been rendered 



* This subdivision contains (1) Stands ; (2) Eye-pieces and Objectives ; (3) 

 Illuminating and other Apparatus ; (4) Photomicrography ; (5) Microscopical 

 Optics and Manipulation ; (6) Miscellaneous. 



t English Mechanic, May 18, 1917, p. 271. 



+ Nature, June 21, 1917, p. 331. 



