618 SUMMARY OF CURRENT RESEARCHES RELATING TO 



MICROSCOPY. 



A. Instruments, Accessories, etc.* 

 (3) Illuminating and other Apparatus. 



Chromoscopic Filter.-j- — This simple apparatus, J. Salkind says, 

 presents transparent and colourless objects coloured on a white back- 

 ground. Ordinary microscopic observation of such objects (living cells, 

 etc.) fatigues the eyesight in consequence of the necessity of detecting 

 the minutest refractive differences, many a detail eluding notice in the 

 uniform greyish-white of the preparation ; moreover, the necessity of a 

 high diaphragm power deprives the objective of the marginal rays so 

 important for the resolution of fine structures. 



The chromoscopic filter, intended to remedy these inconveniences, 

 essentially consists of a glass or celluloid disk of dimensions suitable for 

 its introduction into the Abbe apparatus. This disk is uniformly 

 coloured and is centrally perforated by a circular aperture. Theoreti- 

 cally, observation with the chromoscopic filter holds the mean between 

 vision by transmitted light and that with the ulfcramicroscope. Thanks 

 to the filter, the most oblique rays of the condenser are coloured rays ; 

 they are reflected and refracted by the object and penetrate the objec- 

 tive. At the same time the background of the preparation is colourless, 

 for the centre of the mount — optically homogeneous and of minimum 

 refraction — transmits to the objective only the white rays of the central 

 bundle. Moreover, according to the specific refractive index of the 

 different parts constituting the object, these parts appear either coloured 

 or white. 



It being granted that the conditions necessary for the realization of 

 the chromoscope vary with — (1) the numerical aperture of the objec- 

 tive ; (2) the refractive index of the immersion medium placed between 

 a given condenser and an object-slide of given thickness ; (3) with the 

 refractive index of the object and of its mount-medium — it would seem 

 that the observer should possess a large number of filters with central 

 apertures of varying diameters (or a transparent and coloured iris- 

 diaphragm). But in practice, if a condenser of numerical aperture 1 ■ 40 

 be used, with cedar-oil as the immersion liquid, it will suffice to have a 

 single filter with central aperture of about 5 mm. diameter (the aperture 

 varying a few millimetres according to the colour intensity of a given 

 disk). Correction is performed by means of two star diaphragms super- 

 posable on the filter ; the first, with a dull centre, serves to diminish the 

 brightness of the background, thereby intensifying the colour of the 



* This subdivision contains (1) Stands : (2) Eye-pieces and Objectives ; (8) 

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

 Optics and Manipulation ; (6) Miscellaneous. 



t Soc. de Biologie, lxxviii. (1915) pp. 382-3. 



