INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 889 



some two inches in length, is mounted in this adapter between the 

 radiant and the cone h. This tube has only a narrow central slit 

 opening at each end, which slits being brought coincident in direction 

 with the candle-flame, permit only a thin sheet of light to pass to the 

 object. This restriction of the incident light, though not jiractically 

 important in taking interior angles, shuts out anij question of accuracy. 



The extra traverse-lens o is made a hemisphere, so as to dispense 

 with the object-slide ; traverse-lens h and stage c being removed, and 

 lens b replaced with traverse-lens o. 



The object is mounted on the plane surface of this hemispherical 

 lens under a cover-glass, and all cemented with balsam of 1 '525 index 

 of refraction, The cone k has immersion contact as before, and the 

 cone and all the illumination apparatus can be brought round to 90° 

 of axial obliquity without coming in contact with slide or stage. 



This last-described arrangement of radial-arm and radiant is espe- 

 cially useful in taking air apertures with dry mounted objects under 

 view. 



My method is this : — Selecting a cover-glass of | inch or larger 

 diameter, I place at any marginal part of the cover a little diatom- 

 aceous material, add a drop of alcohol to distribute the same, and 

 by burning off the alcohol the objects adhere sufficiently to the surface. 



This cover-glass is then to be cemented to a slide, not at the 

 centre, but projecting over the end more than half its breadth, the 

 diatomaceous mount being most distant from the slide-end. At about 

 the centre of the slide, another cover of similar thickness should be 

 cemented, so as to bring the slide to parallelism with the face of the 

 back-stage (part of c in figure) when placed with the cover-glasses 

 do\vnward upon it. The slide is then moved to bring the mounted 

 objects into the optical axis, and the objective focussed upon them, 

 with correction in some way for cover thickness. 



Under these circumstances, witli nothing intervening between 

 radiant and object, when we restrict the light that illuminates the 

 object to what can pass through the narrow slits at 

 the ends of the tubes mounted on the radial arm p, ■ 



there can remain no question of the obliquitij of the 

 illuminating rays that give us view of the object ; 

 for rays of no other obliquity are admitted. 



Semi-cylinder Illuminator. — Mr. J. Mayall, 

 jun., sends us the accompanying figure, sliowing a 

 convenient way of mounting a semi-cylinder, or 

 prism, &c., to be used for oblique illumination in 

 the substagc of those stands that are not provided 

 with swinging motion. The mounting permits the 

 semi-cylinder to bo tilted and placed cxcentrically ; 

 in this manner, without inmicrsiou contact, by 

 suitable adjustment, the dry object can bo viewed 

 with any colour of monochromatic light. Placed 

 in immersion contact with tlie slide, the utmost 

 oldiquity of incident light can be obtained on Nobcrt's lines 

 (ruled on the under surface of cover-glass iu air) by refraction into 



