104 THE MICROSCOPE AND ITS REVELATIONS. 



the arrangement of the lenses strongly resembles that used in the Kell- 

 ner eye-piece ( 28); the field-glass of the latter serving as a condenser 

 to receive the cone of rays reflected upwards from the mirror, and to 

 make it converge upon a small Achromatic combination, which consists 

 of a double-convex lens of crown, with a plano-convex lens of flint, the 

 plane side of the latter being next the object. These lenses are of large 

 size and deep curvature; so that when their central part is stopped-out, 

 the rays transmitted from their peripheral portion meet at a wide angle of 

 convergence, and have the effect of those transmitted through the peri- 

 pheral portion of the ordinary Achromatic Condenser. When, on the 

 other hand, this combination is used with a diaphragm that allows only 

 the central rays to pass, these rays meet at a small angle; and the illu- 

 mination thus given is very suitable for objects viewed with low powers. 

 Again, by stopping-out the central portion of the combination; and re- 

 moving the Condenser to a short distance beneath the object, the effect 

 of a i black -ground' illumination (104) can be very satisfactorily ob- 

 tained with Objectives of low or moderate angular aperture. Further, 

 by stopping- out not only the central but also a great part of the periphe- 

 ral rays, so as only to allow the light to enter from a small portion or 

 portions of the margin, illumination of considerable obliquity can be ob- 

 tained. All this can be provided for by a Diaphragm-plate made to 

 rotate at as short a distance as possible beneath the condensing-lens; but 

 as the number of apertures in this plate is necessarily limited, a greater 

 variety is obtained by the use of a Graduating Diaphragm ( 98) for the 

 regulation of the centric aperture, and by making the apertures in the 

 rota ting plate subservient to the other purposes already named, as is done 

 in the arrangement of Mr. Collins (Fig. 75). Still greater variety can 

 be obtained by substituting for the Diaphragm-plate a short tube sliding 

 within the one that carries the lenses; its summit being furnished with 

 a socket into which may be inserted a diaphragm of blackened card or 

 of thin metal, with an aperture or apertures of any shape or size that may 

 be desired. In this manner the diaphragm may be carried up quite close 

 to the condensing lens, which is a great advantage; and when oblique 

 illumination is desired, the light may be transmitted from any azimuth, 

 by giving rotation to the tube carrying a diaphragm with a marginal 

 aperture. The Webster Condenser thus improved (which may also be 

 used in combination with the Polariscope) will be found one of the most- 

 universally-useful accessories with which a Student's Microscope can be 

 provided. 1 



101. Oblique Illuminators. The extremely oblique illumination re- 

 quired for the resolution of the more difficult lined ' tests/ may be pro- 

 vided, as has been shown, either by the employment of a Condenser of 

 very wide angular aperature ( 99); or by giving to the whole Illuminat- 

 ing apparatus (as originally suggested by Mr. Grubb, of Dublin) a posi- 



1 A form of condenser specially adapted for very oblique and also for ' black- 

 ground ' illumination was devised a few years ago by Prof. Abbe of Jena. 

 (" Monthly Microsc. Journ.," Vol. xiii., 1875, p : 77), and has since been specially 

 adapted by him for use with ' homogeneous immersion ' objectives, being fitted 

 to the microscope-stands constructed by Zeiss; but not being found easily appli- 

 cable to Microscopes of the ordinary English models, it has not been taken up by 

 Makers of this country. It seems to the Author, however, that the sliding-plate, 

 by which any degree of eccentricity can be given to the apertures that the opti- 

 cal combination admits of. might, in combination with the Iris-diaphragm for 

 limiting the angle of the pencil, be advantageously substituted for the rotating- 

 diaphragm-plate. 



