ZOOLOGY AND BOTANY^ MICROSCOPY, ETC. 



453 



usual English and American substage or accessory tubes. The tube B 

 carries a third tube C (blackened inside), sliding easily within it. 

 Securely mounted in the latter tube is the spot-lens, which thus may 

 be accurately focused upon the object ; and when once adjusted for 

 any stand, there is no occasion to alter it. If the small tubes be only 

 1/2 in. or 6/8 in. in length, the focusing range is a long one. 



Fig. 60. 



Fig. 61. 



Fig. 62. 



Fig. 60 shows the instrument as fitted to a Microscope which has 

 the fixed tube beneath the stage. By reversing, as shown in fig. 61, 

 the same mount may be used equally well in the movable substage 

 of larger instruments. 



They have also applied the same device to the usual substage 

 Society-screw adapter, for carrying achromatic condenser or objective 

 used as such (fig. 62). 



The inside diameter of the tube C in this case is made 1^ in,, 

 which will exclude very few objectives. It may, of course, be used, 

 as the other, either in Microscopes with fixed stage tubes, or with 

 movable substage. 



Paraboloid as an Illuminator for Homogeneous-Immersion 

 Objectives.* — A. J. Moore attempts "to make two comparatively 

 inexpensive pieces of apparatus take the place and do the work of 

 any first-class wide-angled immersion condenser. These accessories 

 are the ordinary parabola and the hemispherical lens." 



Ordinarily the former is a dark-ground illuminator, but when the 

 aperture of the objective exceeds that of the parabola, the effect is 

 simply that of a dry condenser, in which the central rays are stopped 

 out. But even at its best the light cannot traverse the slide at a 

 greater angle than 41° from the axis ; and it is rarely, if ever, even 

 so great as this. Now, if the light reflected by the parabola could be 

 converted into a glass (or balsam) angle without altering its angular 

 direction, it would be amply sufficient to give light to the objective 

 at the widest balsam angle now used in the best homogeneous- 

 immersion objectives. This may be done by using, under the slide, 

 a hemispherical lens,f whose radius is less than that of the concavity 

 of the parabola, making optical contact by the immersion fluid. This 

 is to be accurately centered and the parabola brought up so close that 

 the hemispherical lens will occupy the concavity. When properly 

 adjusted, it will be obvious that those rays which are transmitted by 

 the parabola impinge normally to the surface of the hemispherical 



* ' The Microscope,' iv. (1884) pp. 27-30 (1 fig.). 



t This was described and figured bv Mr. F. H. Wenham, Trans. Micr, Soc. 

 Lond., iv. (1856) pp. 57-8 (1 fig.).— Ed. 



