412 



SUM3IARY OF CURRENT RESEARCHES RELATING TO 



made to rotate immediately below and in the axis of the optical 

 combination. The carrier-plate moves laterally by rackwork acted 

 upon by the toothed pinion D. To facilitate changing the diaphragms 

 C can be swung out of the axis on the swivel-joint E, as shown in 



Fig. 77. 



Fig. 



Fig, 78. Circular, lune-shaped, and other diaphragms are supplied, 

 which give a large variety of effects of obliquity both in altitude and 

 in azimuth when used with the lateral and rotating movements of C. 

 For black-ground illumination a central stop is placed in B, and 

 Zeiss supplies special diaphragms to be applied at the back of several 

 of his objectives of large aperture which ensure the dark-ground 

 when used in conjunction with this condenser. With objectives of 

 greater aperture than 1 • X. A. the condenser must of course be in 

 immersion contact with the base of the slide. The condenser has a 

 numerical aperture of 1 • 4 nearly. 



Bausch and Lomb's Immersion Illuminator. — This illuminator 

 (of which we have no drawing) is intended " to utilize the full 

 capacity of medium and wide angle objectives," up to 152° in crown 

 glass or 1 ■ 47 N.A. Its mounting is arranged with an internal 

 diaphragm, which is placed directly under the posterior system of 

 lenses, and entirely contained in the tube comprising the mounting, 

 so as to avoid the j)rojection existing with other condensers, and 

 allows the light to enter only from below. By revolving the milled 

 ring of the mounting, the diaphragm is made to pass laterally from 

 the centre to the extreme edge of the illuminator, thereby projecting 

 a bundle of rays of any obliquity, between 0" (central illumination) 

 and the extreme possible limit 1 • 47 N.A. When the diaphragm is 

 at its extreme, a second slit, at right angles to it, giving the same 

 volume of light, is opened by the further movement of the milled 

 ring. The makers add that " the fact that it is used with only central 

 illumination of the mirror, will prove especially valuable to those 

 who do not possess instruments with the modern swinging substage 

 and mirror bar." 



Bauschs Paraboloid.* — Mr. E. Bausch describes a new form of 

 paraboloid in which the hemispherical hollow in the top is left clear, 



* Proc. Amer. Soc. Micr. 1881, p. 88. 



